Crossovers? How do you evaluate if crossover is any good?

I hear Crossover designs are very important to your system. You have several drivers in a speaker and I've seen the curve designs where they slope up and down and when overlaid on top of the curve of another frequency ranged driver they overlap a little bit as slopes go up and down.

So what does all this mean and aren't speakers generally designed similar to other speakers in this area. Is it all about designing the curve? Exactly what is it that makes a crossover better than another crossover?