A shifty is a skateboarding move that is similar to the rewind in snowboarding. It's pretty basic, and can be added to most ollie-style tricks to show some extra flair.
How one does a shifty is simple. When in the air, as in after an ollie or getting air off a ramp, point the board off to the side just by moving your legs. Then, once the board has turned a good portion of the way to perpendicular, you bring it back to straight with your feet again, so you can land. The difference between this and a rewind is that in a rewind, your whole body rotates, while in a shifty, it's just the board and your legs that rotate.
Some people like to call it tweaking an ollie, since it has similar motion to tweaking a grab, but you don't grab it.