You have an amp, a box and inside the box there is a passive
crossover.
It split the signal in a part for the woofer and a part for
the tweeter.
Studio monitors are in general active.
Each driver has its own amp and the crossover is done on the
signal before it enters the power amp.
Onkyo has released a couple of AV receivers enabling you to
implement active crossovers using DSP.
A clear explanation of how it works.
A bit more: active crossover
