Here's the definitions from the APHA website.
Overo
(pronounced: oh vair' oh) The white usually will not cross the back of the horse between its withers and its tail. Generally, at least one and often all four legs are dark.
Generally, the white is irregular, and is rather scattered or splashy. Head markings are distinctive, often bald-faced, apron-faced or bonnet-faced. An overo may be either predominantly dark or white.
Tobiano
(pronounced: tow be yah' no)
The dark color usually covers one or both flanks. Generally, all four legs are white, at least below the hocks and knees. Generally, the spots are regular and distinct as ovals or round patterns that extend down over the neck and chest, giving the appearance of a shield. Head markings are like those of a solid-colored horse--solid, or with a blaze, strip, star or snip. A tobiano may be either predominantly dark or white. The tail is often two colors.
As for the others... Piebald, I believe, is the term for a horse that is black and white, while skewbald is any other color and white.
Pinto is the generic term for a horse with white markings on the body. All Paint horses are pinto, but not all pintos are Paints. There is a Pinto Horse Association registry where you can register any horse with color regardless of breed. Check out their website for specific details.
http://www.pinto.org 
