I've been throwing around the term "core cast," but it's not a universally used term. Kevin Murphy:
Series regulars: People with whom a series has an exclusive deal. Their credit appears in the opening titles of every episode. They may appear in all episodes produced (like Keifer Sutherland on "24"), or they may have a deal to only appear in a limited number of episodes like The Sopranos' Robert Igler or Jamie Lynn-Siegler (even though their name appears in the opening credits)
Guest Stars: These are characters whose credit appears in the body of the show, along with the writing, producing and directing credits. This may be a character who appears only occasionally or a character that's in almost every episode. As a series ages, recurring guest stars may graduate to series regulars... like Kevin "Marshall" Weisman on "Alias."
Special Guest Star: These are guest stars who receive extra favorable credit because of their stature in the industry or because of their seniority on a given series... like Lily Tomlin or Mary Louise Parker on "West Wing."
Co-Star: Often a smaller "walk-on" role, or a larger role performed by an less-experienced actor who lacks the clout to obtain the more desirable "guest star" credit. Co-stars generally make less than guest stars, and their onscreen credit appears in the closing titles... often squished or shrunk in the world of broadcast TV.
There are no hard and fast rules on who is allowed to have his or her own story. Depends on the series. On "Law and Order," the episode is usually all about the plight of a given guest star. On the first season of "Lost," the flashback stories are always about the regulars. "Desperate Housewives" has an enormous cast of regulars, but usually only oneof the four core women is given a storyline.
One nitpick: on L&O, the episode is about the cops helping
the plight of the guest star.
I think you can make a useful distinction between the series regulars, and the narrative stars of the show. The stars of the show are the characters who have their own stories. Series regulars appear in almost every episode but don't necessarily have their own stories. (Star characters may or may not be played by bonafide TV star actors; I'm talking about the function of the character, not whether they hired Jimmy Smits or not.) I think the term "core cast" is useful too. Series regulars are on regular contracts, but you don't necessarily have them in every ep. The core cast would be those characters the audience would be miffed if they didn't see'em in an episode.