Circuit breakers trip when there's too much current going through the circuit (enough to melt the wiring for example), and so protect from short-circuits: hot and neutral touching each other, or a short in equipment.
GFCIs protect against current leaking to ground, by detecting if the current flowing on a hot and neutral leg are different. If they're different, the current must be going somewhere else: to ground, through you, etc. The GFCI breakers do this in the breaker, the outlets do it at the outlet. As someone else mentioned, you can also have other non-GFCI outlets chained to a GFCI outlet, so that upstream outlet is the one that pops if there's a fault.
TL;DR they detect and protect against different fault conditions.
GFCIs protect against current leaking to ground, by detecting if the current flowing on a hot and neutral leg are different. If they're different, the current must be going somewhere else: to ground, through you, etc. The GFCI breakers do this in the breaker, the outlets do it at the outlet. As someone else mentioned, you can also have other non-GFCI outlets chained to a GFCI outlet, so that upstream outlet is the one that pops if there's a fault.
TL;DR they detect and protect against different fault conditions.