Can't be bothered to buy Activision's retail boxed copy of Day of Defeat? You're not alone, and you don't even have to, as it's now been released on the grand old Internet, right here. Plenty of mirrors can be found there for the 173MB Windows version and 170MB Linux release.
So what is Day of Defeat? It's "the latest online action experience from Valve" apparently, and not, for instance, a team-based World War II modification for Half-Life that the developer happened to absorb into officialdom. Each team has several player classes (scout, infantry, sergeant, machine gunner, etc) and specific weaponsets for each, and the goal is to battle as a unit to win through some of the war's historic battles.