Entertainment publicists are PR agents for entertainment companies. They manage music, TV and movie stars, coordinating their public appearances including award shows, parties and TV appearances and interviews, often accompanying them on the red carpet or on press tours. Entertainment PR firms are mostly small firms owned by individual PR agents with enormous personal networks. The most famous ones are Wolf-Kasteller, Rogers & Cowan and Baker-Winokur-Ryder. In general, these firms extract far more fees from business clients than from celebrities, but the celebrities give them exposure and publicity -- typically, celebrity fees are a fraction of corporate fees.
Agents are like accountants. No one particularly likes them, but you've got to have one. They're the only ones that know all the intricacies of the TV/music/film industry, can help you navigate the process of finding a job, and are the ones in the best position to get you what you want.
Most actors, musicians and writers have agents. An agent is usually a very connected person who negotiates deals. Agents typically charge a fixed commission of around 10%. They get writers accepted onto TV shows, they get actors cast, and often they serve as therapeutic sounding boards and friends.
There are primarily two types of agents -- talent agents, who represent actors, musicians and directors, and literary agents, who represent the writers and authors that create the blueprints for the stories that get developed into films. There are countless agencies throughout Hollywood, but there are only a handful that handle A-list actors, directors and writers. The main ones are Creative Artists Agency (CAA), International Creative Management (ICM), William Morris and United Talent Agency (UTA). CAA was the first big mega-agency, started by Hollywood legend and failed corporate executive Michael Ovitz. Endeavor is one of the newest ones, started as an exodus of ex-ICM agents who formed their own group of young power-hitters.
Agents are also famous for handling interference for stars and prominent personalities. For example, if a studio, or a production company wants a star, the agent takes the call and relays info to that star (the notorious "let my people talk to your people.") If a famous person wants something, usually they have their personal assistant (or occasionally their agent) call. If someone is not famous, a good agent can make phone calls to get someone at a studio or production company to take their call. Agents are the people who bring together different people in the industry -- the wheeler-dealers for those who aren't as skilled at networking for themselves. While agencies draws many prospective employees, it takes a certain kind of person with a certain thick skin to stay. It's essentially a sales job. There are tons of agents around and it is highly competitive. The industry abounds with endless stories of individuals who finish law school and start in the mail rooms at talent agencies. However, agencies are one of the best places to start because of the volume of people that one meets and contacts that one gains by working there. It's hard work, even abusive at times, but it is the best way to break into the entertainment world, especially for those with few other connections.