woah sounds loke your starting from the absoult begining...

headunit: is the cd player, these are best for a start get one with rca (type of cable like what you use for dvd to tv connections) outputs this way you can run rca to the amp

Amp: more power to the speakers than the headunit can put out. 2 channel= 2 speakers, 4 channel= 4 speakers, 2 channel bridgeable means all power to one speaker abd so on.

speakers: tweeters are small for high pitched "high's" sound no base, subs are for low notes "base" mids are typically inbetween both

thats basically it, it can go much further but a simple system to sound good doesnt really need thousands of dollars, check on here or craigslist for good deals new you will always pay the most but used you dont really know what condition the items are in or what they have been through.

for you stock speakers in a newer car typically sound decent with a headunit, you can replace them but its kinda pointless unless you hook an amp to aftermarket speakers. in my opinion and experience a stock speaker powered buy a head unit and a aftermarket speaker powered by the same head unit sound pretty close to the same ( i replaced all my stock speakers with alpine type s and factory headunit and think it actually sounds worse) for nice clear stay with a cople 8 or 10" subs unless you want to car to shake get 12 or 15's

i had many systems but my most basic was a 4 channel amp, 2 channels ran to each of my doors to a component set (pair of speaker tweeter and crossover) the other 2 channels were bridged to a 10" sub i had clear mids/highs and a little bump. but i also have had a 1,000 watt 4 channel amp to all 4 speakers with a spare set of tweeters and a 1200 watt amp to a jl 12w7 which was crazy and i ended up selling it because it was too much for my taste