Here are the three main types of systems.

System 1: Basic
Fewer components mean lower system cost and the ability to spend a few extra bucks on the components that really count. The more speakers you have, the more harmful interactions and cancellation of sound waves will occur. Two tweeters and mids that can go down to 60 Hz. Rear speakers are not essential in most cases. Get a good head unit with a clean signal. Get a good quality amplifier; 100 watts per channel should be plenty. The trick is a properly designed enclosure and lots of power. 200 watts or more per sub should add plenty of punch for the bottom end of your system.

System 2: Competition Level
Most people prefer to take advantage of head units capable of high line level voltages for minimum background noise. Equalizers become a necessity to fine tune system's response. You might need to add a high power alternator if the car's electrical system can't handle the extra loads. Wiring and connectors should be neat and clean. The car should also be treated against rattles and road noise.

System 3: SPL
It needs three things: Lots of power, a lot of speakers and a closed place where all that sound can be concentrated. Alternators, capacitors and batteries become essential. Multiple speakers are placed up front, wired in series/parallel combinations to maximize amplifier power. Subs need to have a big cone area and high excursion (Xmax) to be able to move as much air as possible. The amplifiers moving the subs must be able to handle high current demands and to have low impedance capability. Serious SPL competitors replace glass with thick Plexiglas, and reinforce the whole inside of the vehicle with steel, concrete, etc.