Some people say that 'LOL' means 'lots of laughs'. Most people define it as 'Laugh out loud'. It doesn't really make much difference; the underlying sense is the same.
If something is really funny, you might see 'ROFL', or one of its variations:
http://www.computerhope.com/jargon/r/rofl.htm
Alternatively, a slightly cruder abbreviation is 'PMSL':
http://www.computerhope.com/jargon/p/pmsl.htm
'IMHO' causes arguments because some people say that it means 'in my humble opinion' whereas others state that there's nothing humble about their opinion. They use 'IMHO' to mean 'in my honest opinion'. Some people simply use 'IMO', thus avoiding confusion. ('IMAO' means 'in my arrogant opinion'. Unsurprisingly, it's not seen very often!)
'BTW' is 'by the way'.
For more, see here:
http://www.computerhope.com/jargon/chat.htm
Chris