What exactly sets recurring characters apart from the main cast on a TV show?

From a production standpoint it's about credit and pay. Main cast members will be paid more than guest stars. A main cast member will also be paid for any episode in which they are credited regardless of whether they actually appear or not. Usually main cast members are in every episode or almost every episode and recurring characters are only in some of them. But occasionally you do gay situations where it's reversed. In one of the later seasons of Lost for example oh, the character Desmond was credited as a main cast member but was in fewer episodes than one of the recurring members of the Dharma Initiative. In terms of the storyline, nothing really sets them apart and one can move from being a recurring character do a main character, or more rarely vice versa

  Their contract mainly and how much they make per episode. And their Billing on the Credits

plot wise they are there for the main characters to interact with