Rockstar's policy is that you're free to mod the single-player game, but any modding whatsoever of the online game is forbidden, no matter how miniscule. They detect it and it's a potential ban.

That said, they don't publish anything about modding and don't have an official API available. Everything has been done third-party.

The modding has caused real problems in the online play - player in godmode, handing out in-game money, etc. No matter the efforts of R*, they seem to be unable to eradicate it for good. The group of players I previously played the game with, including myself, have all become extremely disenchanted with the game by the online modders.