I don't understand

There are two words i see used fairly often online (especially here) and i don't really know what either of them mean but they sound bad so need to know so i can make sure i am not one of them. The words are "troll" and "catfish" i have no clue what they mean but like i said they sound bad. I know in the state of Michigan i am a troll because i live south (under) the Mackinaw Bridge but that isn't necessarily a bad thing LOL