Original: An undershirt is a general word for a piece of underwear covering the torso, while underpants (in the UK, often simply known as pants), drawers, or shorts enclose the genital region.
Simplified: An "undershirt" is a general word for a piece of underwear covering the torso, while "underpants" (in the UK, often simply "pants"), "drawers", or "shorts" cover the part of the body where the sex organs are.