How sub-satellite system should work - crossovers

Can someone elaborate how a sub-sat system should be put together, and whether I need a seperate crossover to use my little Celestion D100's with a sub or bass unit?
I have never had a subwoofer, and the only experience I have with this topic is that my old roommate had an Ohm N2 passive stereo "bass unit" subwoofer where the amp speaker outputs went straight to the sub, which had built in stereo crossovers, and then the satellite cables went out from the sub to the satellites. Is this how pretty much all relatively inexpensive sub/sat systems work? Thanks in advance!
Also, if there are any recommendations for a sub which would go well with the Celestion D100's (which I am guessing roll off well above 80 Hz, so would need a 'bass unit' rather than a true 'sub') I would love to know.