The Factory is a Terran building that can be used to produce the Vulture, Siege Tank, and Goliath. No research upgrades can be directly performed on the Factory, but a Machine Shop add-on can be constructed onto the Factory that allows various upgrades relating to Factory-produced units. The Vulture can be produced with the Factory alone, while Tanks require the Machine Shop add-on. Finally, the Goliath can only be produced after the construction of the separate standalone building, the Armory.
In TvP match-ups, Factories and Factories with Machine Shops are commonly built in the early game to produce Mechanical unit armies. In TvZ match-ups, Factories are often delayed until mid-game as Terran often rely on Barrack-produced Bio armies early on.
Since many Terran strategies involving Mechanical unit armies, multiple Factories may be built, but the Terran player will only opt for the Machine Shop on a subset of them. In these strategies, Terran players must carefully organize the location of its Factories in constructing its Sim-City and to fit the maximum number of Factories in its main base, but can utilize the Lift Off ability of the Factory to aid in this process.