How can I check if a select dropdown changed

How can I check if a select dropdown changed

Hi.
I am new in jQuery so here it is my question:
How can I check if a select dropdown field changed. In a form I have some select dropdown fields, and I need to check if they were changed or not. I think I should use selectedIndex and defaultSelected but don't know exactly how.
Could someone give me some tips?
Thanks a lot.