There are a few types of speakers out there and what you need to concern yourself with at the big picture level is the location of where your speakers will be. The ones at the front are usually the tweeters that give you the highs and midranges that give high vocal quality and clarity whether you are listening to someone singing or talking on a radio show. On the flip side the speakers placed at the back usually provide bass a little mid to give you the thickness you find in music such as hip hop or techno. This gives you an idea to know what you would be looking for and maybe what your needs and wants are. This then also ties us to the topic of different speaker sizes for your car.
Car speakers are not all built according to universal sizes, many come in different shapes and forms. You must be aware of the measurements and fits for your specific car so that you make the correct speakers choice. Of course with a price anything can be done and in this case, custom cases can be built to fit your speakers to fit properly in your car. In fact for subwoofers (the speakers usually placed in the trunk of your car that provides extra bass) is highly dependent on these cases. Due to the way that these speakers have been built, the case makes a huge difference on even a quality subwoofer.
With quality of course comes with a price tag, and you probably need to ask yourself the question of how much you would want to spend on your speakers. Right off the bat, there are plenty of brand names out there that provide quality speakers, but you should always go to the showrooms to have a real live experience to see which ones sound best for you. In car speakers, the high end ones really often do demand a higher price, so you would most definitely hear the differences between a budget speaker and a high end speaker. At the same time, especially for subwoofers, buying the bigger speaker does not necessary mean the better sound. At the end of the day it is down to a science, and the larger the subwoofer the deeper the bass, but the slower the response time. In layman terms, a big subwoofer is not able to recover fast enough to produce the next bass hit, so keep this in mind when you are buying your subwoofers.
So now you have these factors to help you in making your first initial choices on which speakers to purchase for your car, remember that you have to take into account speaker type, size and price and with that you will be able to make educated choices when it comes to choosing the right speakers for your car.