basic HU will have 1 sets (L&R) RCA out (our clarion has a set)
Higher end one will have at least 2, some 3.

Why 1,2,3 ?

easy ..
1st FRONT CHANNEL (L + R) ,
2nd REAR CHANNEL (L + R),
3rd WOOFER CHANNEL (L + R)

What is the difference between High power input or RCA ?
HPI -> the signal is already amplified by ur headunit build in amp (relatively small power). The amp will take this input and further amplified it.
RCA -> clean signal unamplified by ur headunit.

HPI or RCA better ?
HPI -> normal headunit amp is very weak (mostly 10 ~22w rms max) ... since it is weak, the sound amplified might not be tat nice. Thus it might have amplified some NOISE out from it as well.
RCA -> Signal to the better amp before it is amplified has better cleaner sound.

What does 2 channel 4 channel means ?
1. FRONT LEFT (1 channel)
2. FRONT RIGHT (1 channel)
3. REAR LEFT (1 channel)
4. REAR RIGHT (1 channel)

Total 4 channel setup.

Can stock speaker handle the amp
my rear are running stock

basically the OEM speakers are on paper material. The speaker response to frequency might not be so good. Thus might produce not so clear sound. The amp does help in giving a more power to it to drive the magnet.
SPEAKERS

Basically i devide this to 3 types..
1. Tweeter
2. Mid range
3. Woofer

Tweeter Handle high pitch sound ..
Mid .. obviously ..
Woofer.. low bass sound..

how to make sure it sounds rite .. the rule of thumb for me .. easy ..
let those job suppose to be done by which one .. do by which one ..

How and what does that means ?
I recommended Sothy for component sets (YES MIKE NAME APPEAR!)

what does component consist of ?
2x tweeter
2x 6 1/2 mid
2x crossover..

like i explain .. the tweeter was suppose to handle higher frequency sound ..
if you pass a low frequency sound to it .. it will sound very very bad .. just like u ask a female to talk like a man ..
so here comes the crossover.. the crossover will cut off the lower frequency sound and feed the tweeter for its high freq sound only. Clean .
same goes to the mid .. the mid is MUCH bigger compare to the tweeter.. if u wan it to run high frequency .. means its moving very fast ..
for a speaker that size .. its hard.. if it cannot keep up with the movement .. it will create unnecessary noise.. the crossover will do the same...[/B]