Every improved car audio sound system should have a sub woofer. The sub woofer completes the sound by providing a high efficiency of low frequencies, which is the bass. There are so many types and brands on the market these days that it can be hard to choose the right sub woofer(s) for you. The (s) is because you don't have to stop at just one!
Your sub woofer needs to be placed or held in something, so if you're planning on getting one keep in mind that it will need an enclosure or box (unless it already comes with one).
Bigger is not necessarily better when it comes to sub woofers. If you're going to be pounding your sub woofer every day continuously for long periods of time, then you should choose a woofer with a high maximum RMS power handle. Also, some sub woofers might pound better with certain amplifiers.
Other factors you should consider when shopping for the right sub woofer(s) are frequency response and efficiency (sensitivity). A wider frequency response provides better results on the frequency spectrum, which allows the woofer to pick up a wider range of frequencies. The higher the efficiency (sensitivity), the more the bass vibration can travel. Sensitivity ratings are measured in decibels per watt at one meter or dB/Wm.
Sub woofers can come with a 6, 4, 2 or sometimes 1-ohm impedance for more power out of your amplifier. If you amplifier is 4-ohms and your sub woofer is 2-ohms, your amplifier will automatically only use 2-ohms because it can use more power this way. Sub woofers for cars can come in diameter sizes from 8-19".
You can have as many as you can fit in your car, but make sure you have enough amplifier power to pound them safely. You may need more than one amplifier for multiple woofers. Unless your car is for show, I recommend the sub woofer going in the trunk, because the trunk can act as sort of second enclosure. Hatchback cars tend to hold more bass because the whole car is kind of like a box.
Please note that when you first purchase your sub woofer and get it installed and running, make sure you wear it in before completely blasting it. New sub woofers can sometimes blow because they are not worn in properly. Just leave it running at a decent volume for hours at a time. Also, you should make sure that you have control over the level of bass from either your deck, amplifier or some kind of equalizer because you will need to even it out with the rest of your car stereo.