How does resource manipulation for online games work?

I've always wondered how things like hacks for money and other valuable resources in MMOs worked. Considering the variety of anti-cheat protection used by the companies who made the games I would think that there would be more than one way of achieving a hack like that and each one would be extremely complicated. If you know how they work or have made one before could you enlighten me and everyone else who's asking as to the basic concepts of resource hacks? Thanks in advance!

In some *bad* cases the client is just able to tell the server "Heyo, I just gained a million, pls sync" and the server adds this amount. If the game is done properly then client to server modifications of such value are useless(e.g. Runescape does a proper job at this). It's also possible that you are able to trigger a server event that adds money over and over, e.g. a mission completion that isn't properly checked.