http://www.arts.ac.uk/library/webguides.htm#art
The library of London's Univeristy of the Arts - a section full of links for useful sites on the net.
http://www.1001.org/20th/
modern & contemporary art. search by artist, movement, gallery or style, each artist with portrait, biography, links to articles, essays and interviews; original art, limited edition art prints, photography and poster art, paintings, multimedia and artist's books.
http://www.artchive.com/ftp_site.htm
Good site for researching artists and movements.
http://www.artlex.com/
Definitions for more than 3,600 terms used in discussing art/visual culture, along with thousands of supporting images, pronunciation notes, great quotations and cross-references.
http://char.txa.cornell.edu/
This site has lots explaining the elements and principles of design, different types of media, evolution of fine arts etc.
I hope these are of help to you guys