Can't tell anything about pvp
You have to run up to a mob to hit it (played only melee class), and actually face it. You can't move through, the mob "blocks" you. I'm not sure if players are able to do this to, a lot of griefing potential

But it would be cool for example to protect a flag carrier that way.
When the mob is about to hit you, (check boss animation) and you have to either move out (strafe) or use skills (warriors get a sort of "dive" at lvl 2) to dodge. "Autoattack" is clicking left mouse, and you will swing your sword. Your swings restore "mana", which you can use for your abilities.
You can combo attacks, when you time your skills correctly.
When you for example play lancer, a class with a shield, you have to raise your shield (press C) when the mob swings to actually block. No block percentages in this game.