A pair of trousers is a two-legged piece of clothing you wear on the bottom half of your body. It's easier to climb a fence while wearing trousers than a long dress.
In the U.S., most people refer to trousers as pants, while in the U.K. pants are underwear. If you cut your trousers off above your knees, they become shorts, and if they're made out of denim, you can call them jeans. In Scotland, trousers are sometimes called trews, one possible root of the word. Another source may be the Gaelic triubhas, "close-fitting shorts."