The tech lab is a terranadd-on in StarCraft II. It allows attached production structures to build advanced units, and provides some upgrades for those units.
A structure may not be attached to both a reactor and tech lab simultaneously.
The tech lab provides the advanced facilities needed to store the medpacks used by medics. During the Second Great War, Remedius Medical Systems developed a medpack storage facility that could be placed within a barracks without the need for a tech lab. (This enables medics to emerge twice as quickly from a barracks attached to a reactor.)
The thor attacks an enemy ground unit or structure using the cannons on its back. The target unit is stunned for the duration and takes 500 damage. The thor cannot move or attack when using the ability.
Tech labs only serve to allow the player to build the more advanced units. Upgrades that are normally researched through the tech lab in melee games are instead researched in the armory on the Hyperion and some are already pre-researched (such as the banshee's cloak and the siege tank's siege mode).
Protoss plasma shields have finally given us the ability to fit the tech lab and the reactor into a single structure add-on. This new add-on, known as the tech reactor, replaces the old tech lab and reactor add-ons, and allows us to train two of any unit type simultaneously.
All structures repair themselves to 50% life. All structure fires are automatically put out.
This all-in-one safety system automatically dispatches robotic drones to put out any fires and perform basic repairs on damaged structures. Although these drones are effective at doing basic repairs, they do not have a strong enough AI to perform the more delicate repairs needed to restore a structure past half life.