Defence turrets are very powerful automated projectile or laser firing weapons. They're often protected or armored. It can aim and rotate over many degrees.
Station Defence Turret Edit
Station Defence Turrets are incredibly lethal and shoot any ship that violates station regulations. There's white text on the turret that says "Loitering is a crime." Some of these turrets are equipped with special anti-shield lasers that act like the Reverberating Cascade Experimental Effect, allowing them to damage the generator directly and bypassing any Module Reinforcement Packages in the process.
Ground Based Defence Turrets Edit
There are small ground based anti-ship/anti-air version defence turrets to protect settlements and surface ports. These turrets have small signatures, and ship sensors cannot easily lock onto them from over ~350 meters away.
Anti-Ground Defence Turrets are small turrets for ground defense against SRVs and other ground-based threats. They appear to use kinetic cannons or plasma repeaters to engage threats, rather than lasers, and their engagement range is ~300 meters; the SRV can lock onto them from outside their range and destroy them without being fired upon in return.
Anti-Air Defence Turrets can also be anti-ship and anti-air pictured below. They're located near settlements and surface ports, and are outfitted with powerful pulse or burst laser weapons primarily used to engage spacecraft and ship-launched fighters; their sensors are incapable of locking onto an SRV.
Larger settlements can have defensive 'grids' consisting of many of these turrets; whilst they're not very dangerous by themselves, the combined firepower of an entire defense grid can bring down smaller ships and steadily wear down the defenses of larger vessels.
These turrets are fairly durable (though very vulnerable to Dumbfire Missiles) and will automatically come back online a short time after being destroyed.
Big Defence Turrets have barrels as wide as an SRV. They fire massive kinetic slugs at intruding starships, alternating between barrels for sustained firepower that can wear down any intruders. Like smaller anti-air/ship turrets, these heavy turrets can self-repair.
A Defence Platform is a large turret. The sensor range before a trespassing warning at low security is 728m. The sensor range before a trespassing warning at medium security is 1.6km for ships. However this distance depends on where the defense platforms are placed.