Witches aren't real monsters, but tend to associate equally with monsters and humans. This is possible only thanks to their magical powers, which allow them to defend themselves against monsters, and their being biologically identical to humans, which allows them to blend in without all the hassle that most monsters have to deal with.
While witches can pass for either monsters or humans when they need to, as a result they suffer from some discrimination on both sides. Some monsters think of them as uppity humans who don't know their place, and treat them accordingly. On the human side, whenever a witch doesn't hide her powers she runs the risk of being attacked by religious fanatics or frightened locals.
Though discrimination among humans is rather rare nowadays, as people are far less superstitious and the laws far less concerned with magic powers than it is with the lynch mobs themselves. Although much is made of the witch trials way back when, such things are almost unheard of nowadays. Besides, most of the people that were killed in witch hunts were humans anyway. Any half-decent witch could evade a bunch of humans easily, and even the ones who weren't half-decent could usually find a way out of those stupid witch tests.
Witches are exclusively magic-users, of course, and they specialize in curses. While the witch usually has a quicker spell on hand for when she gets cornered, their real strength lies in more complex magics than your standard attack spell. A skilled witch can utterly destroy an enemy she's never met with no more than a name, a photo, five minutes and a seemingly random collection of amphibian organs. They also have a number of other skills, ranging from potion brewing to magical beast husbandry, which at least makes them one of the more entrepreneurial monsters around, if nothing else.
Witches can be particularly cruel and vindictive in their grudges, and can carry out their revenge with impunity, depending on the witch's individual magic skill. If you do manage to offend one, it's recommended that you take offensive action immediately, ideally to strip the witch of its magical item, which is the source of its power.