What is the difference between operculum and gill slits?

Operculum is for Osteichthyes while gill slits are for Chondrichthyes. Operculum serves as a covering of the fish gills while gill slits are individual openings to gills.

Operculum - Osteichthyes - covering of gills - can close and open Gill Slits - Chondrichthyes - opening of gills - always open