I'm trying to decide which to buy. I don't really know what a crossover does.

 

A passive cross-over operates on the power signal and filters the appropriate part to the appropriate speaker, in laymen's terms. For example, if you have a three way component system, a passive cross-over can be used so that the tweaters only get high frequencies, the mids the mid frequencies, and the woofer the low frequencies. An active cross-over is used on the audio line levels and are generally more accurate, more efficient, and more adjustable.

An equalizer is used on the audio signal and can be used to tweak certain frequencies to compensate; kind of like your bass and treble settings but more dynamic. Then you get into parametric EQs, and the number of frequency bands, etc.