Roll is a tactical-evasive move introduced in the Street Fighter Alpha series.
The move is self-explanatory; the user rolls forwards, how far depending on the strength of the button pressed. Ken first used this move, and Akuma is also able to use it, though his version is called Zenpou Tenshin (the major difference being Ken's roll happens on command but Akuma will take a slight hop before rolling forward). This move is also used by both Ken and Akuma in the Street Fighter EX series.
This is an excellent and quick move for countering an approaching opponent, throwing off the aim of their attacks, or simply messing with them. This attack oddly does not fill up the Super Combo Gauge at all.
Ken also has a "fake-out" or "pretend" roll, by doing a quarter circle forward and taunt. In this, he will roll forward, fall down, and get up. This is good for mindgames, but one must remember Ken is fully vulnerable during it. He can do this infinitely.
For more sprites, see Roll/Sprites.