WASD according to the Wikipedia:

"WASD is a set of four keys on a QWERTY or QWERTZ computer keyboard which mimics the inverted-T configuration of the arrow keys. These keys are often used to control the player character's movement in computer games. W/S control forward and backward and A/D control strafing left and right. Primarily, WASD is used to account for the fact that the arrow keys are not ergonomic to use in conjunction with a right-handed mouse.

Some gamers prefer the WASD keys to the arrow keys for other various reasons, including the fact that more keys (and therefore, game commands) are easily accessible with the left hand when placed near WASD. Left-handed mouse users may prefer using the numpad or IJKL with their right hands instead for similar reasons.

After being popularized by first-person shooters, WASD became more common in other computer game genres as well. Many of the games that have adopted this layout use a first-person or over-the-shoulder third-person perspective."

There are other alernatives such as WQSE, ESDF (sometimes preferred because it provides access to movement independent keys for the little finger), IJKL (common in browser games because employing the arrow keys woud make the window to scroll and thus hinder gameplay), and of course the unix-based HJKL. Why do I blog this? documenting different styles of interaction, it's intriguing to see how the arrow key configuration evolves and mutates.