Warfare is complex, and shooting something is not as simple as pointing your weapon at it and pulling the trigger. You need to decide how precisely to aim, and how many shots to fire. There are three types of shots usable in X-COM: Snap Shots, Aimed Shots, and Auto Shots. All weapons can perform the first two shots (save for the Stun Rod and the Blaster Launcher), while only some weapons can lay down sustained fire with Auto Shots.
A standard shot from the hip. Accuracy is medium for the weapon, TUs used are lowest of all shot types. All X-COM reaction fire shots are Snapshots, as well as some Alien reaction fire.
A dedicated shot at the target involving precise aiming. Highly accurate, but costs many TUs. Aimed shots can never be used for reaction fire.
A three-round burst of fire laid out of the weapon. Accuracy is low, TU usage is moderate. Not all weapons are capable of automatic fire. Aliens can use Auto Shots as reaction fire.