The Hosts are an unseen, mysterious occupying force that invaded and occupied Los Angeles and elsewhere (at least New York) during the event known as The Arrival. These invaders are in command of 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. bloc.
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.
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 a Host or 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.