.bashrc vs. .bash_profile

[Matthew S-H]

Sorry to ask such a simple problem, but i looked through the first  
100 results on google and found nothing.

What is the difference?  Under what situations does it read one over  
the other?
I know it depends on if a shell is interactive or net.

But when is a shell "interactive"?  And what does it mean to be  
"interactive"??
Is it referring to interaction with the user, other processes, or what?


~Matt