Originally Posted by bigaudiofanat

Not to make anyone sound stupid but active is for advanced users. Instead of letting a crossover or the amp set your crossover points. You set and control everything including the crossover hrz and all from the head unit. It allows for a lot more control over a system with a lot higher end components and amps. I personally was going to go active but never got to that point. But after about 2 days of running you can make a system sound amazing if it is installed and tuned right.

Alos the cool thing about alpines are if you do not want to do all that they have something called "imprint" which is a device which will setup your system almost as good as you could do by hand with a laptop and a mic.

In normal terms active means the most control over a car audio system.

This might help as well
Active vs. Passive: An attempt by me to explain - Car Audio Forum - CarAudio.com