...what exactly do they do? And do i need an amp, or is it recommended, for component speakers with power range 25-100 watts RMS and peak power handling 400 watts?

 

Amps are a good thing they defiantly help to push the speakers. Crossovers protect your speakers from trying to produce frequencies that they can't handle. It limits the tweets to do their thing and the woofer to do its thing.

 

So they will eliminate bass to the woofers and tweeters? or do i still need to get something else that does that?

 

Yes, they will protect your speakers, use the cross over if they came with your speakers. You should not need anything else.

 

It depends on whether you're talking about active or passive crossovers, and how they're applied. Technically, a "crossover" is nothing more than 2 or more frequency attenuation filters combined together.

When you buy a "component" set (mids & tweets), most will come with a set of passive crossovers. These will simply keep the bass out of the tweets, and highs out of the mids, but they WON'T block the bass from your mids.

Most amps for subwoofers have a built-in low-pass filter (meaning it allows the lows to pass through), such that only the low-frequencies are amplified and sent to the sub, high frequencies are filtered out. Amps for component speakers typically contain built-in high-pass filters (pass the highs to the amp), that WILL keep the low-bass out of you mids and help prevent them from distorting while allowing them to play louder than they would when they have to try to reproduce the low-bass signals in addition to the midrange.

It's possible to purchase or build your own passive high-pass filters to keep the bass out of the mids, but the components needed to create filters for frequencies under 100Hz are fairly large & expensive, so it's much better to use an active filter built into the amp.

I'd suggest bi-amping, one amp with active low-pass filter to power a sub, another amp with active high-pass filter to power your components, with the included passive crossovers separating the frequencies between your mids & tweets. That's a very common, cost effective configuration that produces good results.

 

^^^^
That is a good explanation.