The Hosts, also known as the Clicks, are a largely unseen, mysterious occupying force that invaded and occupied Los Angeles and elsewhere (New York, Switzerland and the UK for instance) during the event known as The Arrival. These invaders are in command of the Global Authority, the Transitional Authority and Homeland Security. Their origin, and their agenda are unknown.
They are also sometimes called the Raps, apparently a reference to raptors (the Homeland Security, the LA and Global Transitional Authority logos all feature a stylized bird of prey). Some senior members, including Proxy Governor Alan Snyder, of the Transitional Authority have direct contact with them. It's not clear how much contact takes place, how frequently, or in what circumstances.
The Hosts have inspired a popular new religion in the L.A. Colony and other colonies.
In Season 3, we learn more about why The Hosts came to Earth. Katie Bowman and Will Bowman figure out the truth after bringing the gauntlet to the resistance outside the wall. The Hosts ran away from their planet, after being attacked by an unknown alien species. They came to Earth looking for allies against this alien species. Which may be the reason why the Hosts needed both Collaborators and labor forces.
The Hosts are actually an extraterrestrial group of alien robots. The Hosts have a machine-like body structure. This body structure contains an orb that holds their consciousness. Its believed that they may have either been made by another alien species or that they may have uploaded their consciousness into the orb. There are only a few hundred Hosts. The drones and other machines are automated and operate as a hive mind. According to Alan Snyder, he says that the Hosts told him that they're trying to protect humanity from the Hosts' enemies. It is theorized that the Hosts believe that authoritarian dictatorship can protect humanity, and use them as allies. It is also theorized that the Hosts has invaded the entire world, so that the Hosts can use Earth as a military base of operations against their enemies, and the Colonies are being used as safe zones, which is why they set up the Walls around the cities.
There's very little confirmed information on The Hosts, and none on their intentions, but some information has emerged:
- Phyllis, during a phone conversation on the night she was killed, remarked that "Their perception of time is different." From the context it's difficult to see how this could refer to anyone but The Hosts.
- Alan Snyder, in the episode "YOKNAPATAWPHA" told Will Bowman that he had met them and that the experience made him realise how insignificant humans were.
- Alan Snyder hinted, by his manner with Katie and from what he told Will that there was some contact between The Hosts and humans before The Arrival. This was later confirmed in "Eleven.Thirteen" when Snyder was seen meeting with a Host just before he was made Proxy Governor of the LA bloc.
- At the end of the episode "Zero Day", when The Resistance board the metro carriage they've just bombed, Katie and Eric Broussard see what is either a humanoid figure in a protective suit or a humanoid robot. Pictures in the Stills Gallery below.
- In the episode called, "Seppuku", Noa reveals that one of the Hosts defected from the others because it doesn't agree with the other Hosts' agenda. However, in "Sierra Maestra", the Host defector was actually a prisoner of war, and it being a defector was propaganda.
- In the episode called, "Ronin", it is shown that the Host is a robot and the scientist says "Our Host is online", and it was later revealed that there's a moderate faction of alien hardliners amongst the Hosts, and they were overruled.
- In the episode called, "Sierra Maestra", Will and Katie discovers that the Host is a machine and it finally reveals its true motives; the Hosts fled and escaped from their enemies, before coming to Earth, and they enslaved humanity, because they needed humans to use as both allies and labor resources to build their defenses against their enemies. This could be the reason why the Hosts need Collaborators.
The Hosts have invaded and occupied many countries throughout the world (see The Occupation) starting with the event known as The Arrival. Proxy Governor Alan Snyder told Beau that he has seen them and makes contact with them. Helena and Phyllis do have direct contact with them.
Harsh reprisals take place when resistance forces have successes. At least one city was destroyed (Dallas was turned into a "sheet of glass") when two Hosts were killed there. 
Helena Goldwin travels to Switzerland to speak to the Global Transitional Authority soon after she is made Governor General. At the meeting there appear to be many countries represented, sufficient to require simultaneous translation services, similar to those provided by the United Nations Interpretation Service with interpretations being providing, via headphones, to those present at the meeting.