Hi there, i´m sure that almost of your know 2d controls, those ones that follow the camera axis and not the player one.
Ad example: you press forward (or up), in a 3rd view camera but the camera is looking from top (like rpg or strategic games), while you press up, the player moves forward facing him (that is 3d),
in 2d, if you press up, the player goes above the screen (isometric 2d screen) or moves far (3d screen).
For 3D controls if you press down, the player go backward.
In 2D controls if you press down, the player go near the camera or down.
Examples of games using 3d controls : Resident Evil 1,2 and 3, Tomb Raider
Using 2D controls : Super Mario 64, rpg games, silent hill 3 or 4 have this method too.