Same difference as "song" and "favorite song".

A friend is someone you like to spend time with and a best friend is the one you trust with your secrets.

HaleyHeart is right. A "best friend" is exactly that--a friend you feel closest to and care the most about. Literally, the "best." Usually this term only encompasses one or a few people. As far as the term "friend" is concerned, you will get a variety of answers. Websites like Facebook and other social networking sites are redefining the meaning of this term. Source: American English (self)

Best Friend is usually referring to the friend you love or care about the most, and also a friend who you're with more often than average friends.