I just started watching a movie and it listed the cast and then showed 'guest stars' and listed additional cast members. What constitutes a 'guest star' as opposed to the regular cast of a movie?

I’m wondering how guest stars differ from the regular cast or if there is any real distinction at all between them. Does anyone have the inside scoop on this mostly unimportant question of mine?

(It’s 1:05 a.m. at work and I have a lot of extra time on my hands so it was important for me to ask this question for no substantial reasons whatsoever. Thank you in advance if you choose to answer it.)