Roman satire is the only literary form that the Romans can be said to have invented (although, to be sure, we find elements of it in the Greek writers as well). It is the ridicule of vice or folly. It necessarily has a moral quality to it. Human folly is not simply criticized, it is mocked through exaggeration and clever wit. It is usually also humorous, although it does not need to be. Satire can use parody, but it is not the same thing. Parody uses some feature of a person, thing, or event to ridicule it.
We can think of satire as a response to imperial power, although it certainly need not be that. It is an answer, a way of showing that "the emporer has no clothes", or at least that those who have power aren't all they think they are.