The "Keating 5", originally the Keating 4, is the collective nickname for Annalise's Law Students and main characters of the show.
The Keating 5 work with Annalise Keating at her law office as interns in order to gain extra experience in order to make them better at whatever law career they choose when they graduate law school. Some students such as Bonnie Winterbottom have remained with Annalise even after she graduated from Middleton University and became her personal secretary/assistant. To help make the students more competitive and cause them to work harder, there is a trophy which the chosen student can use to get out of any exam and instantly pass it without actually taking it.
The first four students (Laurel Castillo, Asher Millstone, Michaela Pratt and Connor Walsh) were chosen because of their work that they did on the trial case in class. The additional student (Wes Gibbins) was chosen was because of what had happened to the student's mother and Annalise felt it necessary to include him in her "Keating" group for her part that she played in the student mother's death.
Original "Keating 4" ("She Hates Us")
The only Keating 4 we know of in addition to the current ones are the Keating 4 of 2005. Annalise chose Bonnie, Kennedy, Xavier, and Noah from her Criminal Law class to work with her in her in her personal cases. Bonnie Winterbottom had therapy with the husband of Annalise, Sam to help her deal with her problems in childhood. Of the 4 bright students who helped her in cases, Annalize decided to stay with Bonnie as her right hand. In addition to this, Sam helped Frank Delfino by managing to convince Annalize that he will hire him as an assistant. When Frank was hired, he had a tendency to flirt with the other students and even sleep with them from time to time.