The T-800 unit was developed and produced by Skynet following Judgment Day, originally August 29th 1997, to be used to eliminate all remaining humans who survived the nuclear fallout. In order to do so, the primary function of the T-800 is to assimilate into human society and assassinate key members of the resistance. Designed with combat in mind, the T-800 units are extremely durable with a 120 year power supply and the ability to withstand damage from near enough any kind of ballistic firearm as well as relatively small explosions. In fact, even when immobilized, they can try to re-route power from their battery or harness it from external sources to become operational again.
Unlike the earlier T-600 model which had plastic flesh, the T-800 has more human flesh grafted onto the endoskeleton thus making it far more effective at blending in. However, there are still large drawbacks. Primarily the fact that the flesh is covering a huge robotic body meaning that even when in disguise, the T-800 is still much bulkier then most humans. Also, while the new grafted on flesh means the T-800s can bleed, sweat and generally feel human, dogs are still able to tell them apart from real people thus leaving them open to attack, as seen in the 'future flashback' scene of the original movie.
In the movies, the official title for the character Arnold Schwarzenegger plays is the Cyberdyne Systems Series 800 Model 101 which, according to James Cameron, means that while he is a T-800, the 101 model looks like Arnold. This means that the Terminator who infiltrates the human camp in the 'future flashback', played by bodybuilder Franco Columbu, will be a T-800 but have a different model number entirely.