Afk'ing is where the monsters are constantly aggressive at you so that your account wont log out, even if you dont move the mouse or keyboard, because you are constantly in combat.
Most aggressive monsters become unaggressive after you kill a ton of them. The reason bandits are good for afking is because they never become unaggressive due to your god item.
At your level, I dont think there are any good npc's to afk.
The bannability (new word I think) of afk'ing is low. But if you afk for hours it could look like you are autoing. Also, randoms make it look like you are autoing. Like when I am afk'ing at monks with my def pure, the drunken dwarf is constantly there. So when he kills me with his constant 1's, it makes it look like I am an autoer.