The Companions are the supporting protagonists of the long-running TV show (and related multimedia franchise) Doctor Who.
They are a group of specific people chosen (either by fate or by the Doctor himself) to accompany the Doctor on his journeys through time and space.
In many ways, the Companions are the true heroes of the show. Very often, the Doctor finds himself unable to deal with the villains he faces (either because of being physically unable to, or because his pacifist leanings forbid it), leaving the Companions to defeat the villain and save the day.
Companions are often women. Male Companions are rare, but they do tend to exist (the most notable ones being Captain Jack Harkness, Rory Williams andBrigadier Lethbridge-Stewart.
The first season makes it more evident that the Companions are the true protagonists of the show, and two spinoff shows in the Doctor Who franchise (Torchwood and The Sarah Jane Adventures). However, most of the time, the Companions share protagonism with the Doctor.
The Companions serve as the moral support of the Doctor, and the ones who keep him calm and stable, with the Doctor becoming unhinged or depressed without a Companion.
Companions are often surrogates for the audience, with the audience learning more about the story and the universe through the eyes of the Companions.