The Assassin Droid is the sniper unit for the Trade Federation and later for the CIS in Star Wars: Battlefront. They are mainly used to pick off enemies from long-range, their rifle has a double-zoom feature which allows them to get a clear view of the enemy at longer ranges. Their primary weapon is a high damage, slow-firing Sniper rifle that can easily kill in a single headshot and cause heavy damage otherwise but it fires slowly and its shots leave a very visible tracer, meaning that in order to gain reliable kills, you have to get a head shot the first time you fire. Otherwise, the tracer will give away your position to the enemy which can give him a chance to duck into cover and even fire back at you. In Star Wars: Battlefront II, the Assassin Droid and its counterparts do not get a targeting reticule when not zoomed in-- but this is no hindrance to experienced players, who can become quite good at 'hipfiring.' (Equivalent- Scout Trooper, Rebel Marksman, Clone Sharpshooter).
The Assassin Droid is called the Droid Sniper in the first two Clone Wars campaign missions in Star Wars: Battlefront I.
The Assassin Droid has a red skin in Star Wars: Battlefront I. This symbolizes that the droid belongs to the Trade Federation. In Star Wars: Battlefront II however, the red skin is passed to the Assault Droid.
The Assassin Droid sports a regular battle droid skin for the Rebellion of Theed and during the Battle of Naboo.
The Assassin Droid also sports a sand paint job on Tatooine and Geonosis and a grass paint job on Felucia and Kashyyyk.
Its appearance in Star Wars: Battlefront II is similar to the basic Battle Droid, except they're more filthy.
In the films, the red vested battle droids are called Security Droids who have very different functions than Assassin Droids.