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The car audio you use will have the definitive say in how your system will sound. There are many different types of speakers available. A single speaker can be used to reproduce the full range of sounds but it is not ideal. If the speaker is too large it will have problems reproducing high frequencies which require rapid movement of the speaker. If it is too small it will have problems reproducing low frequencies which require large amounts of air to be moved. Because a single speaker cannot reproduce all sounds accurately numerous speakers are used each of which duplicates sound in the frequency range it was designed for. A speaker called a tweeter reproduces high frequencies generally above 2 kHz.

Tweeters are small and lightweight so they can respond rapidly. Very little power is required for powering tweeters because they are very efficient. Woofers are the exact opposite because they usually require large amounts of power to really move air. Woofers are meant to produce sound at frequencies below 250 Hz and often just below 100 Hz (in the case of subwoofers). Because a woofer must move considerable amounts of air they are usually large with typical sizes of 10", 12", 15" and even 18"! On the other hand tweeters are usually very small ranging in size from 1/2" to 2" in size. Typically, tweeters larger than 1" in size cannot respond quickly enough to sound good  and are too directional. In between are midrange speakers which handle the frequencies between the woofers and tweeters. Further separation can be done but is usually inessential and just complicates the crossover which must separate the full audio signal into Jl Audio multiple parts for each speaker.

Things to look for:
Power Handling: Just as with amplifiers, RMS or continuous power is critical here. Some manufacturers will declare very high power handling figures but they are usually for very short peaks only. Granted music is not continuous but the continuous power handling gives you a much better impression of how much power a speaker can really handle. For tweeters and midranges, power handling is not as important since it does not take much power for them to play loudly. For woofers though a rough match should be made between the woofer and the amp driving it.

Sensitivity: This is a very important spec for a speaker. It gives you an idea of how loud a speaker will play given a certain input power. If a speaker is insensitive then it will require more power to play at the same volume level than a speaker that is more responsive. Figures between 85 dB and 95 dB at 1 watt RMS at 1 meter are general. If you use anything outside of this range you may have problems matching the output levels of the speakers relative to each other. If you're going to run speakers off of a head unit then try to get speakers with higher input sensitivities since head units normally do not have much power.
 
Physical Size: You must pay attention to the size of the speakers you choose. Tweeters are very small but need to mounted where they fire nearly directly at you or they may not be heard properly. Some tweeters have better off axis response than others. If you will not be on axis with the tweeter when you audition tweeters in a store listen to how their sound variates as you move around them to see if they will work in your car. Midranges should fit in the door or dash spaces provided or you will have to do some cutting or fabrication. In general the bigger the woofer the larger the enclosure required to hold it. Some woofers are better optimized for little enclosures than others (Kicker Solobaric, JL Audio W6 for example). Make sure you have enough room in your trunk or hatchback for the woofer. Kickpanels for midranges and tweeters or coaxials typically offer superior imaging than locations in the door however the soundstage is sometimes lower than when you have the tweeters mounted high in the doors or on the A pillars.

How Do Car Audio Speakers Work?
Moving Speaker Speakers are air pistons that move back (on the negative cycle of the signal) and forth (on the positive cycle), creating different degrees of air pressure at different frequencies. The amplifier (either separate or built-in your radio), produces electrical impulses that alternate from positive and negative voltages (AC).  This current reaches the voice coil inside the speaker, forming an electro-magnet that will either be repelled, or attracted by the fixed magnet at the bottom of the speaker.  The voice coil is joined to the cone, moving it back and forth, creating sound.  The surround (rubbery circle that joins top of the cone and metal basket) and the spider (usually yellow corrugated circle joining bottom of cone to magnet) make the cone return to its original position.