Wolfram Research
Wolfram Math World: Fermat's Theorem
This site summarizes Fermot's Theorem, there are links to related topics on the page, some prior understanding of modulus is suggested.
Wolfram Research
Wolfram Math World: Fermat's Last Theorem
Very detailed and comprehensive site from MathWorld.com about Fermat's Last Theorem. This site has good explanantions and links to related topics.