I've never kept rainbows so I can't give you actual experience based knowledge but from what I know of them they're very hardy and peaceful fish and that's a good combination.
So, being peaceful they can be kept with many other peaceful fish. They're okay with a wide range of different water conditions. Probably best to keep them in more neutral water than really soft or really hard water though. Temp wise they're good with 77F - 82F. Lots of people keep them in planted tanks, though this isn't required. They like fast moving, oxygenated water. They''re not picky about food.
So yeah, this is the basis. There's lots of rainbow species out there though - what species are you specifically interested in?