You can keep anything in a 20g - There are people on here keeping SPS, LPS, Softies Clams, in a 5g tank. You just have to have the right equipment. Corels you are pretty much open to A LOT! Fish on the other hand, you are limited. You can't stick a Sohoal Tang in there. It all depends on your equipment.
Do your research on here, look through the threads. get some T-5 lights, a nice Hang On Back SKIMMER and weekly to biweekly water changes, You can keep a lot of different corals. Get about 20-30lbs of live rock for you filtration, since you don't have a sump this will work good. Read through the "NANO" forms. A lot of information for smaller tanks.
Good Luck! Read, Read, Read!! And Take Your Time.