I want to play Garry's Mod and Team Fortress 2 at the same time so when I get bored of one I can easily alt tab to the other and when I'm needed back on the RP sever I like to join I can go back then as well.
Steam doesn't have such limitation, but you can't play two games that use Source Engine at the same time.
Yes, you should be able to just alt-tab out of one game and open the other. This is a bit resource-intensive, though, so you may suffer some performance loss if you don't have a pretty powerful computer.
I haven't tried this with Garry's Mod and TF2 specifically (I don't own the former), but there is typically no such restriction. In fact, earlier today, I accidentally launched two copies of DEFCON at once with Spiral Knights updating in the background!
Steam does not limit how many games you can run at once, however, you can only run one source based game at once. This is a limitation of the engine, not Steam.
Yes, assuming that both (or all but one) of the games you want to run simultaneously are reasonably gracious about being switched out of context. But that's an issue with the games in question, not Steam.
Steam's only limitation in the mix is that it will only start one game at a time; as soon as the "preparing to launch XXXXX" dialog vanishes Steam hands over control to the game itself and you can start a second.
As others have said, only if it's not two Source games. But, many non-source games can be opened together (and you can have ONE source game and another non-source game). Steam doesn't really care, it mostly depends on the games. But don't try to open two really memory-destroying games at once. Unless you're using a supercomputer.