Slowroll;
To reveal one's hand slowly at showdown, one card at a time, is to slowroll anyone else who thinks the pot might be theirs. This is usually only done with a winning hand, for the purpose of irritating other players (well, some people do it innocently).
Slowplay;
To slowplay is to underbet a very strong hand (i.e., to play it slow, except that when used in this way it's made into one word). The purpose of slowplaying a hand is to give other players the chance to make stronger second-best hands, and also to conceal the strength of your hand. Instead of betting early and risking the loss of future action, slowplay means checking and calling. It's of course best to slowplay when you have a hand that no one is likely to actually catch (e.g., four of a kind). Slowplay is not the same thing as check-raising, but the two strategic options are similar in that both are often intended to trap more money in the pot in situations where you are fairly sure you will win.