Premise and fact are essentially interchangeable on the GMAT. That is to say that premises are to be taken as facts and facts are premises and not conclusions.
The conclusion is what the author is trying to convince someone of: meaning an action to take or an opinion to adopt, etc.
The best way to tell a premise from a conclusion is simply this: Conclusions are supported by the premises, where premises are just statements, usually without support.
My old favorite is "All females play tennis. Mary is a female. Therefore, Mary plays tennis." The first two statements are premises. Notice how they are not supported? How do you know that all females play tennis? You must accept this without evidence.
The conclusion is supported. Notice that the conclusion is the last statement "Mary plays tennis" this statement builds on the other two. It is supported by the other two. The conclusion is supported and the statements do the supporting.
