Technically, a cog or gear has teeth designed to mesh with the teeth of another gear, or at least something else with teeth. A sprocket has teeth designed to mesh with a chain, or something perforated.
But sprockets often get called cogs or gears, and on a bicycles the front sprockets get called chain-rings and the other terms reserved for the rear sprockets.
Technically, a cog or gear has teeth designed to mesh with the teeth of another gear, or at least something else with teeth. A sprocket has teeth designed to mesh with a chain, or something perforated.
But sprockets often get called cogs or gears, and on a bicycles the front sprockets get called chain-rings and the other terms reserved for the rear sprockets.