Turrets are the primary weapons on your ship. They are attached to High Power Slots and come in three general varieties:
Projectile, Auto cannons and artillery cannons;
Hybrid, Rail guns and blasters;
Laser, beam and pulse guns.
Each requires a special type of ammo, with Projectile turrets requiring shells, Hybrids requiring charges, and lasers using a frequency crystal.
Typically, projectile weapons are used by the Minmatar, Lasers are used by the Amarr, and Hybrids are used by the Gallente and Caldari, although each can be used by any race's ships.