When booking a speaking role and/or titling it on your resume these are the terms you will typically see.
Series Regular: The leads of a television series.
Recurring: An actor who appears in multiples episodes but it not a lead.
Guest Star: An actor who supports an episode. A guest star will normally perform in multiple scenes and have a character arc in the episode. Often times, the actor playing a guest star role will appear in the opening credits. A good example of guest stars are the actors who play the killer in a crime drama.
Co-Star: An actor who supports a scene or two. For instance, a grocery store clerk or bank teller who helps a principal actor. Co-Stars normally have lines, but on occasion a casting director will cast a non-speaking co-star.
Lead: The main character in a film.
Supporting Lead: A smaller part then the lead that still plays an intricate part of the story.
Supporting: A character that simply pushes the story forward for the sake of the lead or supporting lead characters. The equivalent of a television co-star.
Contract: The series regulars of the soap opera for a particular season or seasons.
Day-Player: The equivalent of a television guest-star. Perhaps has a larger role in an episode.
Under 5 aka “U5”: Under five lines. Comparable to a small co-star.