Both Melsc and Without Wings have said sensible things here and you should definitely look at those posts, in addition to forestall some questions here is a bit of a generalised summary of the english, math and science courses.
English
Standard- this seems to be for people who have very little interest in studying english. An area of study in Journeys is undertaken in the HSC which is common to the Standard and Advanced courses and constitutes 40% of the final mark in the HSC. This Area of study will also contain some comprehension, creative writing and an essay. You will also study 3 modules which are specific to Standard which look at how language effects our perception, a close study of a text, and how certain texts reflect aspects of society. In the preliminary component of the course your teacher will design her/his own modules and Area of Study in order to prepare you for your HSC year. Contrary to popular rumor propagated by both students and teachers, it is not scaled differently than Advanced by UAC, however the course guidelines seem to have a somewhat negative effect on the trend of marks achieved by students in past years with only 3 people achieving band 6 in this course - ever.
Advanced- this course is fairly similar to the Standard course in its outcomes but is designed to be slightly more challenging for the average student as it will cover a range of mediums such as prose, poetry, film, shakespearean drama etc. The Area of Study is held in common between the Standard and Advanced courses so see above for details. You will also study 3 modules which are specific to Advanced which look at how context and society shaped the growth of texts, an indepth analysis of a text and the critical theories and schools of thought associated with it, and to see how our perception is manipulated by textual techniques. In the preliminary component of the course your teacher will design her/his own modules and Area of Study in order to prepare you for your HSC year. Advanced must also be taken in order to do Extension.
Extension 1- if the Advanced English course looked at communication, you could say that this course was looking at literature, it has 9 electives of which you only get to specialise in one, however the electives are grouped into three modular divisions and these are: genre, ways of thinking, language and values. The first one, genre, studies the categories texts fall into and how they have evolved and changed according to the social values of the audience and composers to give us different conventions such as; the nosy old lady of the manor house solving who did what, the young apprentice who must go out and save the world while learning along the way or the all knowing detective such as Holmes or CSI's Gill Grissom. The second one, ways of thinking, looks at a school of thought or trend in text so to speak such as postmodernism or globalism and this module could be said to be one of the more philosophical in the HSC. The third, language and values, is essentially looking at how and why things are said the way they are and you could look at for example the social roles of the genders. When you sit the final exam for the course you will recieve 2 questions with the trend being for one analytical and one creative writing task. In the preliminary component of the course your teacher will design her/his own module, one of the more popular prelim modules has been to explore the gothic, alternative choices have been postcolonialism, romance and "the hero".
Extension 2- this course is all about you. Your decisions, your visions, your work. It also should not be thought of in the way many people think of Extensions, don't do this course just because you are a talented Advanced & Extension 1 student, do it because you want what this course has to offer *specifically*. In this course you will compose your Major Work which can take the form of a short story, poem, script, video, critical essay and websites among others. You will also be required to write a Reflection Statement in order to justify and explain your work and this will be also handed in with your Major Work to be marked by the Board, your internals would also consist of a proposal- an essay where you state your intents and purpose, a viva voce where you must in an interview justify your work as both a product and the processes you've undergone to get it there, and then a report which is just that - a report. You will also have no exams whatsoever in this course. Sounds bludgy? It isn't, this course will stress you to the bone at some point or another and whats worse is that while its perfectly possible to get 100% internally, the Major Work is so subjective that you never really know what mark you do or don't deserve and alot of people end up feeling they got a raw deal because of this. There is also no prelim component to this course ie it is for the HSC year only.
Maths
General Maths- this course is for people who generally do not wish to study maths past highschool or want to do maths that is more suited for vocational purposes. You will look at the maths of finance and its applications, you will study basic statistics and probability, as well as some relatively simple geometry, trigonometry and algebra. Also while it is true that this course does scale down, this doesnt matter in the long run if you perform well.
Mathematics- this course will provide a basic background for most sciences in university requiring a background in maths and is somewhat more difficult than General Maths. The skills of algebra and arithmetic are expanded upon, along with geometry, probability, basic graphs, trigonometry, logarithms, exponentials, series( numbers that follow a certain sequence), quadratics and calculus(this gives you some extra graphing techniques and allows you to determine velocity and acceleration among other things).
Mathematics Extension 1- this course expands on the ideas presented in the Mathematics course as well as introducing some completely new concepts. Some of the things covered are how to apply geometry to a circle, polynomials(shortcuts on how to find the solutions of equations with a higher power than 2), induction, inverse functions, the binomial theorem( a way to predict what the coefficient to a term might be in bracketed term containing 2 terms such as (x+y)to the 2) as well as new techniques and ways of looking at the work done in Mathematics. It would be best to do this only if you find you can cope with the Mathematics course as some people do find it fairly challenging.
Mathematics Extension 2- this course is designed to challenge you mathematically, it can be hard and it can get easier too, but it will seriously kill you if you unless you either a) study your arse off or b)are naturally talented or c) enjoy maths. You will basically study the core ideas of math(well highschool math, but anyway) and you will find that it is basically about presenting you with problems and getting you to do them in any way that is possible - attack it from all sides till you get what you need. Obviously this course extends concepts from Extension 1, so your marks should be up there accordingly if you want to do this course, you also should most definitely not do this course because of the scaling, if you don't do well in it then you wont get scaled up and you will waste all that energy on units that won't count because of poor performance.
The Science Subjects(well the most popular ones at least.)
Biology-Biology is a study of the way living things function and why. Within the core of this course you will look at ecology, drugs, organ transplants, the heart, the kidneys, blood, urine, DNA, epidemiology, the theory of evolution and the photosynthesis of plants just to name a few things. In the options you will gain the opportunity to study one of the following; how creatures communicate including such things as the structure of eyes, ears and vocal cords or biotechnology, where you will investigate the technologies involved in biology such as artificial insemination and transgenics or you will look at the biochemistry of photosynthesis in plants or you will study genetics in depth, looking at how we pass on traits and features or you will study how humanity has evolved from his prehistoric state to what he is today by look at things like anatomy and bone structure. This course will blend quite well with the outcomes of Earth&Environmental Science, Chemistry and Senior Science. Also in conjunction with Chemistry it is considered a suitable background for many scientific studies including Medicine, Veterinary Science and Pharmacy.
Chemistry - Chemistry is a study of how elements and compounds interact to form the complex reactions in the world around us, as well as those we produce ourselves. In the core of this course you will look at the classification of elements&compounds, atoms, the atmosphere, valency and bonding, the history of metals, the mole concept, hydrogen bonding, solubility, water's relationship with heat, hydrocarbons, renewable fuel, batteries, nuclear chemistry, acids in the environment, esters, the synthesis of ammonia, water pollution just to name a few things. In the options you will gain the opportunity to study one of the following; industrial chemistry which includes studying the roduction of sulfuric acid, the making of soaps and detergents and the Solvay process or an indepth study of corrosion and its prevention or a look at the biochemical reactions of movement or a study on how forensic investigators employ chemistry in a variety of ways for example blood analysis or the chemicals involved in paints and dyes as well as other areas of art. This course would blend quite well with Biology or Physics and does require a limited degree of mathematical capability. Chemistry as a subject is generally considered reccomended background for the lion's share of science subjects at uni not to mention a great deal of engineering courses, and when taken in conjuction with Biology or Physics it provides an excellent background for such courses as Medicine, Veterinary Science and Pharmacy.
Physics- Physics is a study of motion, force, waves and fields whether they be magnetic, electrical or gravitational. In the core of this course you will look at velocity and acceleration and learn how to explain them scientifically, you will look at how gravity affects the motion of objects, how sound works, the orbits of planetary bodies, motors and generators, how current flows, the photoelectric effect, and cathode rays among other things. In the options you will investigate one of the following; the physics involved in geology or the study of physics as it relates to medicine such as ultrasound, x-rays and MRI scans or you will study the physics of the stars and the galaxy or you will study atomic theory and quanta and so on or an indepth study into electronics and transistors. This course would blend quite well with Chemistry, Cosmology, Engineering Studies and Mathematics, it also requires a limited degree of competence in maths. Physics is a subject which is a useful foundation for a large amount of science and engineering courses in university, and when taken in conjunction with Chemistry is an excellent background for Medicine.