Marines are the Terran's basic ground infantry armed with guass guns and StimPack tech.
Due to their ability to attack both ground and air targets, Marines hold the line until more specialized forces arrive. Recently, they have been issued combat shields to increase longevity on the battlefield. Though not as high tech as plasma shielding, they can still take a reasonable amount of punishment from small arms and lower strains of Zerg.
For more information see Marine on the parent site