Astromech units are the utility droids tasked to carry out information retrieval assignments and perform complex computer maintenance and repair work throughout the galaxy. They are usually short, stout and barrel shaped, and they move across surfaces on two or three legs equipped with treaded rollers. Astromechs usually communicate in electronic whistles and beeps, which require translation by protocol droids. R2-D2, who served alongside both Anakin Skywalker and his son Luke, is the best-known astromech droid in the "Star Wars" saga.
Protocol droids resemble their humanoid masters and aid in matters of etiquette, rituals, social norms and translation. Protocol droids can be programmed to understand up to six million forms of communication, including the purely mechanical languages used by astromech droids such as R2-D2. The best-known protocol droid in "Star Wars" is C-3PO, the human-cyborg relations specialist built by Anakin Skywalker before the rise of the Empire.
Battle droids were the backbone of both the Trade Federation and Separatist movement before and during the Clone Wars. Created to look like the skeletons of the Neimoidians that commissioned their manufacture, battle droids are relatively inexpensive and are built in large numbers. Battle droids have only limited processing capability and can't make decisions; their numerical superiority and heavy firepower are offset by their dependency on centralized control from computers
Destroyer droids, or droidekas, are heavily armored military machines designed and built to compensate for the limits of battle droid performance and firepower. Destroyers have heavier blaster armament shield-generators that protect them from energy and projectile-firing weapons, as well as the ability to change from a fighting stance into a wheel-like travel mode. However, destroyer droids are also dependent on centralized control from computers.
Probe droids are intelligence-gathering robots used by both the Sith and the Galactic Empire to seek Jedi Knights and Rebel bases before and during the Galactic Civil War. Probe droids, which are also known as probots, possess high-end sensor suites, blasters for self-defense, communication links, espionage programming and a self-destruct mechanism to prevent capture by an enemy. Probe droids can hover over any terrain on repulsor lifts and are launched into deep space via hyperdrive pods.
Interrogator droids are globe-shaped devices that float in midair with the help of repulsorlifts and are equipped with a wide array of torture devices and drug injectors. Interrogator droids were once illegal before the rise of Emperor Palpatine, but the Galactic Empire's secretive nature and desire to crush any opposition resulted in their eventual creation in Imperial security facilities. An IT-8 Interrogator Droid was used during Darth Vader's interrogation of Princess Leia Organa aboard the first Death Star.
Medical droids are Class One units -- the highest class of droid in the galaxy -- and are programmed with all the medical knowledge available to their creators. They are designed to offer the best medical care possible to patients of various species. Some medical droids can perform operations so skillfully that they leave little or no scarring. Some medical droids, including the 2-1B models used by the Rebel Alliance, seem to acquire empathy and compassion after years of long service with sentient life forms.