A company hires an Advertising Agency to get their product or service before the eyes of the public. The Ad Agency, along with the Company, decides what kind of commercial will be effective. The Agency then hires a Production Company to produce the commercial. The Production company hires a Casting Director to provide the actor(s) for the commercial. The Casting Director then sends out "breakdowns" of the characters being cast.
These "breakdowns" actually are sent out by a service called "Breakdown Services" and are only available to licensed Talent Agencies.
Then the talent agents submit photos, resumes, and sometimes "demo-reels" to the casting directors. Then the casting directors call in the actors that fit the roles they are casting, usually many many actors are called in.
As Talent, it is very important to build and maintain a positive and professional relationship with the Casting Directors in your areas, as they are the ones who decide who is seen by the Client.
The Casting Director contacts Talent Agents, or the Talent directly, and arranges to have appropriate Talent come and audition. This is usually done by having each Talent perform all or a portion of the script of the commercial and it is recorded on video tape for review by the Production Company, the Ad Agency, and possibly the Client. At this point, the Talent will be informed and must agree with the terms of the job, both how much it pays and the day(s) it will shoot.
When the choices have been made, the Production company contacts the Casting Director, and then the Casting Director contacts either the Talent or their Agent to "book" them for the project.