In my understanding.
Xylem are the main tubes in the vascular bundles of plants (the other being the phloem) and thier prime purpose is to transport water and disolved minerals up from the root hairs to the leaves. they do this using a mechanism called evaporation (transpiration)-tension-cohesion. this may sound complicated (or it may not) but basically it means that: water in the leaves (which carries any disolved minerals) is given energy by the sun to evaporate (TRANSPIRATION), this creates a negative pressure (TENSION) which pulls more water molecules from below up to the leaf, then (this is a bit of chemistry aswell) because the water molecules are polar (have negative and positive ends) they attract one another and move up the xylem together (COHESION). Hence the name evaporation (transpiration)-tension-cohesion. this process is a form of passive transport (i think thats what its called) which requires no energy.
Where as Phloem are the smaller tubes in the vascular bundles that transport sugar or glucose from the leaves (or any other part of the plant that contains photosinthesising cells) to any cells in the plant which require sugar. So, basically the glucose is broken down into disacharide sucrose which is a smaller molecule that can be transported more easily and can penertrate semi-permeable membranes (for entering cells at thier destination). This disacharide sucrose is then transported along with various other molecules (amino acids, nitrogenous compounds, and other nutrients) to its destination via active transport (a form of transport that DOES require the expenditure of energy ATP(Adenizene Tri-Phosphate). The sugar soloution that is transported is commonly known as sap (you should know what that is).
So, i guess thats kind of a lot to take in. and it is more complicated than what i told you so please cross-reference this infomation with info in the text book. it explains things in a lot more detail there. and i also may be wrong in a couple of parts.
But basically just remember that Xylem caries water up using passive transport, and phloem carries sugar in whatever direction to where it is needed using active transport.