Barracks is one of the most important Buildings in the game. They produce all melee Infantry, as well as some Ranged infantry and Medics. They are first built in the Prehistoric Age and continue until the Nano Age (Space Age for Empire Earth: The Art of Conquest).
It's better if you build two or more barracks, because that way you'll get a much bigger army in a short time. It is also better if you scatter them throughout your town/civilization because you will be attacked from more than one side. It changes its appearance every two epochs. If you choose the Babylonian civilization you will always have the Sampson available, despite the epoch upgrades.