These two websites are insanely full of information. Maybe not the best place to start learning, just from information overload, but it will always be interesting:
Knots on the Web (Peter Suber)
Roper's Knot Pages

For learning, I'd check a library or bookstore. There's about as many books on knots as webpages, but the easiest to use and most useful I've found is The Ultimate Encyclopedia of Knots & Ropework, by Geoffrey Budworth. Got the first edition (with very minor errors, being a first edition) for $15 at Powells.

J