Difference in floc and clarifier?

Can someone explain the difference in floc and clarifier? They seem to do the same thing.

Thanks in advance

They do basically the same thing but in different ways.
Floc causes particles in the water to clump together and then fall to the bottom when the pump is turned off where they are vacumned to waste (cannot be done with a cartridge and some DE filters)
Clarifiers cause small particles to clump together into larger suspended particles that the filter can remove more efficiently. They can be used with any kind of filter but have the most benefit with sand and cartridge filters. If you overdose on the polymeric clarifiers they can have an opposite effect and cause the particles to form a colloidal suspension that the filter CANNOT filter out.
IMHO, except under very special circumstances neither is really needed in a properly maintained pool!