h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

UK:

Trousers are pants and pants are trousers, i.e., from belt height to ankle. (Male and female usage)

Shorts are short trousers or pants i.e., from belt height to knee or higher. (Male or female usage)

Underpants are under garments for males to wear under pants or trousers. (strictly male usage)

Very recently (last decade or so) has seen the blurring of the term underpants with knickers. Prior to this period knickers were strictly for female usage. I think that this derives from the development of "new man" and other attempts at political correctness!

These are current usages and don't take into account the history of archaic derivations of pantaloons and the like.

USA:

They have changed a number of English terms to distinguish themselves from the mother country and its tongue.(a direct result of their revolution and evolution)