The Auto turret is your basic Base defense, more effective in early game the Turret is a long lasting stationary defense that is placed within a restricted area around your base. its fast attack and high damage makes it strongest early and semi-mid game. in the late game upgraded marines or mercs will blow the turrets away before they even finish buildings. the auto turret cost 10 minerals and can be upgraded with extra damagae and armor with the structure defense upgrade at the command center.
for optimal defense make sure you strategicly place your turrets where as few of the enemies as possible can touch it while still unloading its full dmg and saving your base. If under heavy attack early game and you can afford Shift+clicking/Hotkeying your turrets so you can spam build them is more effective then doing them slowly one at a time. just like the marines the Turrets are very powerful in numbers!
The Auto Turret can also be used with the Raven, a unit purchased from the Unit Shop. The Raven's auto turret has a 5 turret capacity limit with a slow charge cooldown. This was added due to abuse of the Raven's auto turrets shortly after the release of the Unit Shop. During the week when it was released, it became a widely used and "cheesed" strategy to purchase Ravens, and fully upgrade auto turret range, and throw auto turrets along the walls of people's bases, this derived the name "Raven Arena" where almost every game there was someone using this strategy.