A best friend is when if you were e.g. Your'e in a situation you could depend on them to tell the truth, as if they were a friend you would still trust them to tell the truth, but couldnt know if it was the real truth. You will also spend alot of time with this person and you would pretty much always be their first choice.
You: I forgot my lunch at homefriend: Aww, heres £5=)best friend: Good you were getting fat anywayyou: I failed the testfriend: Aww, maybe you'll do better next time=)best friend: High five!! I failed as well!!best friends relate to each other more and are more likely to tell the truth and they couldnt offend you=)