i dont know really. i have time to get this stuff so a budget isnt really a massive priority. i was thinking about like maybe spending 600 for the basics like the h/u and front rear speakers but thats a variable. i got no idea about amps or why people have more than one or this thing u call 'splits' and all this crazy jazz.
amps, they accept a low power, high quality signal from the head unit and make it louder. near on all headunits have their own amps but they arent very powerful to put it lightly. like my car has a head unit and 4 6.5" speakers, the sound is ok at normal volumes but is **** when its loud.
an amp also gives you options to clip or limit the range of sound a speaker plays, so you have have say the fronts doing just high range ( vocals symbols etc ) with the rears doing mid to low ( guitars drums beat etc )
a "split" is a speaker that has the high range tweeter as a seperate unit to the speaker itself, this allows the speaker to be a little bit better in quality ( doesnt have a pole poking out from the middle ) and also allows the tweeter ( the 20c coin sized thing ) to be mounted wherever you like. they are good up front
using two ways or threeways in the back is acceptable because at that distance you cannot make out the tweeter from the main speaker.
two way and threeway is a speaker that has the tweeter mounted in the speaker itself
two way->
Split->
that square thing is a crossover, or controller box to let you tune the speaker and tweeter ranges
hope that helps a little.