The Aliens are the main antagonists of the National Geographic mockumentary When Aliens Attack, which is speculation about what would happen if aliens invaded Earth.
The Aliens are represented by machines that come in five forms: robots, drones, harvesters, carriers, and the mothership. The robots are approximately three stories high and resemble a barrel on digitigrade legs. A green "eye" on the barrel serves as its main weapon, and it has a smaller gun below the "barrel." The drone is about the same size, but instead of legs, it has wings shaped like scythe blades. The harvester is a roughly cylindrical craft ranging from 3-10 stories in height; the bottom is entirely occupied by a tractor beam. The carrier is a triangular craft perhaps half a mile across, and serves as a launch base for other aliens. The mothership is a 50+-mile sphere with several "fins" sticking out the back and a series of rods in a depression at the top. These rods serve as docks from which carriers are deployed.
From what is seen, Alien society is centered on the harvesting of chlorophyll and protein from planets. The Aliens are seemingly led by the Mothership.
All Aliens, except for the carriers, are armed in some respect. The robot has a large "eye" that fires a green orb, most likely made of plasma, and has a small gun that fires a smaller plasma orb. The drone has a similar plasma cannon, but does not have a gun. The harvester uses its tractor beam to harvest plant and animal life, whereupon the chlorophyll and protein are extracted. The mothership has a massive EMP generator, and may have other weapons.
The Aliens are machines and do not feed. However, harvesters collect chlorophyll and protein as fuel for the Aliens, presumably using some for themselves and bringing it back to the carrier, which in turn uses some of that fuel for itself, distributes some more for the robots and drones, and finally takes the rest to the mothership when the invasion ends.
The homeworld of the Aliens is unknown and abandoned. For all intents and purposes the mothership is their homeworld.
The Aliens are extremely durable and/or agile. A drone was able to effortlessly dodge missiles from a pair of F-18's before destroying the planes with similar ease, and the carrier above Washington D.C. survived unscathed after being hit by eight nuclear warheads with a total strength of 1.5 megatons (which is because it spends extended periods in space being battered by space dust and cosmic rays). The only external weapon shown to harm an Alien was a rail gun used to shoot a robot at a range of 100-200 feet. They are, however, much weaker on the inside, as a primary tactic of the human rebels was to "feed" the harvesters bombs. Even the carriers are vulnerable in this regard, as several strategically placed bombs were able to cripple them.