Great website for learning form by watching the clips of the specific exercises.
Also this website is good for learning about the muscles of the body in more detail.
I personally have used this website many a time in the past for exercises I was unsure how to perform. I have it under my favourites.
It contains all the main exercises for each muscle group arranged in to specific sections.
http://www.exrx.net/Lists/Directory.html
Enjoy!