basic questions

I've been messing around in the Gimp over the past few days and I think I've done a pretty good job learning on my own (so to speak...I made the userpic I'm using. I'm fairly proud of it xD)

So I feel kind of stupid since these seem like things I should be able to find on the net (I searched before I posted, honest!) but first of all when making icons:

1. how do you make borders or rounded edges? 

2. what, exactly, do brushes DO? I mean, I understand the concept, but when I was messing with them all they seemed to do was make my picture look like a 5 year old made it  -____-

3. where or how does one make cute little hearts (sometimes see them in speech bubbles) they look like they are outlined like how you do with text (I know how to do that)...I'm guessing it involves adding it as a layer xD thats all I know. 

Any help would be super appreciated! Thank you :)