Simple explanations of how crypto works.
The ever-excellent Khan Academy has produced a very nice and short series of videos explaining how cryptography works. Anyone who understands basic high school arithmetic can follow this. If you have ever been interested in the science of codes, ciphers, breaking them, etc. this is worth a look:
More in-depth treatments of cryptography can be found here:
And for the truly hard-core some of best books on crypto are: