if you're using a component set (mids + tweeters), you will definately need a crossover for best results.

A crossover basically filters unwanted frequencies, but to make you understand better, a crossover will split the sound output from your HU. The tweeter basically is there to output high frequency sound, however your HU's output is going to send all sound (low, mids and high) to it, and this will distort its output. Same goes with mids. So, what a crossover does is it filters out the mids from reaching the tweeters, and filters out the high's from reaching the mids. Set up properly, you won't notice where the seperate sound channels, but since each equipment is only getting the signals meant for it, there is less distortion and clearer sound quality.

ain't no ICE pro, but thats what i know