Rocks are super-tight poker players who wait around with the patience of a tree for monster hands. When those hands finally roll around they bet them strong and everyone folds because they know that a rock only plays with the mortal nuts. A lot of rocks are elderly gentlemen who look like they're itching to tell you about how they survived the war in Korea. Their boredom and endless patience flows into their poker game, but the real weakness is that they are utterly predictable.
Since we're generalizing about people who play poker it is interesting to note that very few female players are rocks. They're either super-aggressive (maniacs), very tricky (good players), or very passive (calling stations). Poker is all about quick reads and confounding the expectations of those readers, so if you're a female poker player there could be a good deal of short-term profit in behaving like a rock at your local card room. Male players make lots of assumptions about their female counterparts, and behaving like a rock is unlikely to be one of them.
