What is Stemming?

Stemming means matching on the "stem" (or root) of the word rather than the exact word.

Many of the popular search engines support Stemming. For instance if you search for pet lemur dietary needs, search engines like Google & Live Search will also search for pet lemur diet needs, and other related variations of your terms. 

Live Search now has a new feature that I think is a form of visual stemming. You can use images to find other images.

Search in SharePoint also supports Stemming.