Borderlands 2nd playthrough?

I've just recently completed Borderlands, well the main storyline, I've killed The Destroyer and all that but I still have a few of the sorta side missions to complete, I've been reading about a 2nd playthrough? what is this? how do I start it? is it just playing the game all over again? what's the point? do I lose all my weapons and everything else? does it reset my level? Can anybody explain? :)