A red shirt is one Unlucky Bastard; also known as Cannon Fodder, Acting Extra or, Damn, I wish I had gotten a better role, but my mommy couldn't pay my way through acting school.
The UFP - Unbridled Fornication of the Poor - claims to oppose the death penalty. In reality, they merely strap a red shirt to their convicts and wait for the nearest passing alien to decide that he wants a snack.
They are a running joke among the flag officers who frequently bet on how these unlucky guys get killed. It had even spawned the creation of a special rank for anyone who survives called the Fleet Ensign which will be rewarded to any Redshirt who makes it back alive.
At one point Starfleet attempted to do the right thing and try to save the captains the effort of copy-and-pasting letters to their families and tried to dispel the bad luck of red shirts by making everyone wear them. While this policy was effective in saving the captains from their tedious paperwork it had resulted in the deaths of said captains (Spock, killed by radiation, and Terrel, killed by own phaser, being the foremost examples.) Starfleet began to lose entire ships at a time (The Boseman and the Enterprise-C were the most recognized), during this period eventually causing them to go back to using other colors.
The ultimate irony? The actual red shirts themselves are indestructable.
See also: Blueshirt
Real life RedshirtsEdit
The Washington County Super Band: no one knows why the shirts were red but there are over 400 of them so its pretty cool when they show up (about once a year, playing You're a Grand Old Flag).