Might is used to show possibility or a suggestion, and so is could. They can also both be used to express annoyance that something hasn't been done that was previously possible. "You could have told me we were out of toilet paper!", "You might have asked before taking my clothes!" In general the main difference is that might is also used to politely ask for permission. Might is also used less frequently by younger speakers, and has a gentler feeling to it. Just like "may" and "can", the present tense forms, the difference is subtle.