WOW! What a broad question.
Basements are damp because of water or moisture making its way into the basement. It's that simple. HOWEVER, the 'fix' that's needed to keep the water and moisture out of the basement is another matter.
If you have a 'specific problem/question', and want to share it w/us for advice, we'd be glad to help. I can't imagine that you're just curious as to why a basement it damp.
Jay J -Moderator
PS: If you have a question, PLEASE give some background about the problem ...