is one of the best known people in the field of cryptography
He's the author of the symmetric cipher called blowfish. He is also, with John Kelsey, Doug Whiting, David Wagner, Chris Hall and Niels Ferguson, the maker of twofish. Twofish is another symmetric encryption algorithm, and an AES candidate.
Further, he is the author of the solitaire encryption scheme, which gives the user good crypto with nothing more than a deck of cards. Solitaire was also featured in the excellent book Cryptonomicon, by Neal Stephenson, there known as Pontifex.
Schneier is however not limited to designing encryption algorithms. He has also done great work in the field of pseudo-random number generators (aka PRNG). The PRNG he and John Kelsey have thought up is called Yarrow, and uses a combination of a symmetric encryption algorithm and a secure hash to make a very secure PRNG.
Together with Mudge he cryptanalyzed the VPN protocol used in Microsoft Windows (called PPTP).
What he's probably most known for tho, is his authorship of the probably best known cryptography book, entitled Applied Cryptography. He's also the editor of Crypto-Gram, a free montly newletter you can subscribe to over at www.counterpane.com.
Not to mention that he's the founder and CTO of counterpane systems.