In PDAs and other situations where interface uses touch interfaces, a gesture is a pointer motion pattern that is interpreted as a command.
Some examples: In Blender, if you push down left mouse button and draw a line, you go to grab mode, curved line will get you to rotate mode and V-shaped line gets you to scaling mode.
Also, some other programs on "desktop" machines, even some games, are using gesture-based interfaces -good examples of programs that use gestures include Opera and Mozilla (through Optimoz), and one game that uses gesture-based interface is Black & White.
Gestures are a bit of evolution (though not necessarily always in obvious direction) after pie menus.