Component speakers will come with an outboard crossover. With a component set you will get midwoofers, tweeters, and the crossover. Most manufacturers of audio equipment will offer a component set. Check out
www.sounddomain.com for some good info. There is a forum on that site that has tons of info.
Most amplifiers have some sort of crossover built in. Amps have a low pass filter for subs that will roll off high frequencies. Amps also have a high pass crossover to prevent low frequencies from going to coax or component speakers.
The crossover included with component sets will further filter the frequencies; sending highs to the tweets (3500Hz and up). Check out
www.bsae1.com for more info than you really want.