Could means have the ability to do something. It doesn't mean its desired or likely. "I could jump off this building" (doesn't mean I want to, just that I have the ability to) Might means there's a chance of it happening. You're unsure if it will happen or not. "It might rain tomorrow"

http://www.m-w.com/dictionary/might http://www.m-w.com/dictionary/could There are lots of different situations in which you can use these, so I can't just give a short answer to this one. Read the description and you'll get an idea of how it's used.

I think "could" used to ask, and "might" used to answer often.