Bi-wiring is just running separate speaker cables from the single terminal on your amp to the two sets of terminals present on bi-wire compatible speakers. Doesn't have to be special "bi-wire" cable--you can run two sets of normal speaker cables if you want and just stack up the spade connectors on the amp end. (There is usually a jumper plate connecting the two sets of terminals on the speakers that should be removed).
Bi-amping is running the separate terminals (generally representing the "high" drivers and the "low" drivers for the speaker) on the speaker off separate amplifiers.
In the former case, the "split" occurs after the amp and before the speaker. In the latter case, its a line level split after the preamp and before the amps. If you bi-amp, you can do it horizontally (one stereo amp for the low end, one stereo amp for the high end) or vertically (one stereo amp for the Left, one stereo amp for the right). Maybe I have that backwards, but you get the idea.
Jim, for the record, you don't necessarily *need* an active crossover to bi-amp--I'm bi-amping a pair of ML Prodigy speakers without an active crossover b/c, while MLs are set up for bi-amping, ML strongly counsels *against* active x-overs. For Bigthistle's info, there is a metaphysical debate as to whether I'm getting the full benefits of bi-amping. The debate (I'm simplifying) being balancing putting another active component in the signal path (passive bi-amping) versus allowing your amps to function better by having them only responsible for a smaller bandwidth of freqs. to deal with (active bi-amping).
I can attest, however, that I do get some benefits--I can hear them.
If you aren't using an active x-over, you need two sets of line level outs to feed the two sets of amps. Some pre-amps have two sets. For pre-amps that don't, you can get specialty line level Ys that create two. Or, you can get a funny solid metal block (for RCA connections) that turns one female RCA into two. That's what I've done.
