splits are just speakers ... they have a seperate tweeter and woofer ...
for sound systems u can basically split all the sound into 3 different sections, very low frequency, mid-range, and high frequency.
the base is very low frequency, u use a sub to do that. maybe below 150Hz
midrange between low and high i can't remember the specific frequencies
splits seperate the mid and the highs, small tweeters make the high freqs and a woofer (usually 6.5 inch) makes the mids ... u can still get OK base out of woofers but they definitely dont' sound as good or low as a sub.
eg of splits http://www.everythingav.com.au/product.asp...?productid=1013
just an example .. this particular set probably isn't very good but it shows you everything. those funny looking circular blue things (usually they aren't shaped like that) is just the crossover, ie it just filters out the different frequencies to each speaker.
hope that helps