mully - as stated above,
LOL generally stands for
Laugh
Out
Loud (and sometimes variations on a theme) and it is used all over the internet - sites like this, e-mails, instant messengers etc.
However,
GAB is specific to a few members on this site and isn't used anywhere else in the same context. So you needn't concern yourself with that one.
Some of the main ones can be found by clicking
here. This site also contains some of the more common smilies used at the bottom.