Hate speech is speech that expresses hate for a certain group of people. (No big surprise there, huh?) Some hate speech advocates violence against the members of the group, some does not.
But the issue gets more complicated. Should hate speech be considered protected, free speech? Conservatives and Libertarians seem to think so. This appears to be related to the conservative belief that an individual's actions are all conscious choices, so even speech that encourages violence can't be said to cause violence.
Most liberals seem to believe that hate speech which does not advocate violence is protected and that which does is not. This appears to be related to the liberal belief that an individual's actions can be influenced by society, therefore strongly advocating violence makes one responsible for it when it occurs.
Of course, there are people who feel that disapproval is the same as hate, and define hate speech in terms of that, but that's a writeup for another node.