the proof used the conjecture made by Taniyama-Shimura involving elliptic curves and modular forms (w/e those mean) (see source)

"The greatest difficulty was in proving that the Taniyama-Shimura conjecture was true. This is the contribution made by Andrew Wiles, and the final stage in establishing Fermat's Last theorem." (see source below)

http://mathforum.org/dr.math/faq/faq.fermat.html