From Command and Conquer Wiki

Barracks are a structure desinged to provide living quarters for soldiers as well as store their equipment. Generally simple, they are designed to hold large numbers of soldiers comfortably, with most of their facilities located underground.

They are the basic infantry training building of the USSR, the Allied Forces and GDI (Nod uses the more symbolic Hand of Nod). In each case, the building was a simple prefabricate, designed to be set up as quickly and efficently as possible, with upgrades usually consisting of the building being adapted to withstand the hazards of the time.

For a list of infantry trained in Barracks see list of infantry trained at a Barracks.