I thought this video explanation of Public Key Cryptography: Diffie-Hellman Key Exchange [or, if you prefer, Diffie-Hellman-Merkle key exchange] was unusually clear. Secure key exchange is really important, because exchanging keys securely with someone is an essential prerequisite to creating a secure communications channel with them.
This video is great for people who want an intro to one of the central ideas in modern cryptography:
OK, there was a little math in there, but not so much.
Dan Boneh’s online crypto course is very good for those who want a more in-depth treatment.
http://www.coursera.org/crypto/