Tobiano is one of the four pinto genes that are found in the horse. It creates a pattern of white that usually consists of large, smooth 'blobs' of white. The white usually crosses the spine on the horse, the face usually doesn't have blue eyes or large white markings, and the legs usually have white socks/stockings. The tail is usually bi-colored.
Minimal tobianos may just have white on a few legs and a bit of white along the neck/spine. Tobianos with more white will usually have white that leaves large, smooth patches of color. Tobianos with a lot of white will have 'sheilds', or areas of color usually found near the flank, chest, and on the head.
All pinto patterns are best shown than described, so here are some examples of tobianos and inkspots (click on the photo for the credits & MORE information):
© Annamaria Tadlock