Mount & Blade: Warband multiplayer is PvP only. Unless you play a certain mod that is not. But it's the best PvP experience that I have had.
I would suggest beginners to join siege or team deathmatch game modes first. In siege, the top team is defenders. As defenders, you don't have as much pressure on you. You have time to observe what's going on and experiment with different equipment and classes. Although, cavalry usually isn't useful in sieges
There are not too many game mechanics facts to learn. If you have played singleplayer, you know the basics of combat and that things usually work more or less as you would expect them to work in the real world. However, even though the game is easy to understand, it's not easy to master; improving your skill and trying out different things is fun in this game. And most fun of all is teamplay.