Beat 'em ups is a term which refers to video games which pit a fighter or group of fighters against many underpowered enemies. Gameplay usually spans many levels, with most levels ending in an enemy boss. If multiple players are involved, players generally fight cooperatively.
It is often useful to characterise gameplay as either 2D (largely characterised by the player walking only to the left or right) or 3D (characterised by full movement in the implied horizontal plane, sometimes also with a button for jump). Graphics can likewise be categorised as 2D (with sprites, sometimes with an isometric or parallax effect) or 3D (polygons), or hybrid (e.g. sprite characters in front of polygon backgrounds, or vice versa).