A friend, to me, is somebody I can talk to for twenty minutes straight and not dislike. From there on, if we can do each others favors and express the most remote signs of liking each other's company, then we're pretty much friends. Shared interests are excellent, but not required to find someone you enjoy the company of.
A best friend would be someone I truly hold dear, someone who understands and accepts who I am, and whom I can trust with everything and know that they trust me.