Traditionally there are only two styles of car speakers: coaxial and component. Each has its own benefits and drawbacks.
Coaxial speakers are designed to so that all the individual speaker drivers play sound from the same location (or axis). This is the most common type of speakers found in cars because it is easier and cheaper for a car manufacturer to make. They are also easier to replace because the installer only has to worry about removing the speaker from a single location.
Component speakers have separated drivers that mount separately. The best example of this is a car that has a mid-range speaker in the door panel and a tweeter in the A-pillar. Component systems use a external crossover to filter the audio frequencies to the mid-range and tweeter. Component speaker systems are becoming more popular as they can offer better sound quality and a more realistic sound stage.
Speakers come in many sizes from a smaller 4″ driver to a larger 6″ x 9″. The first step in replacing your car’s speakers is to determine what size speakers fit in the factory locations of your car. This is best done by using a speaker fit guide found on the internet (such as the Sonic Electronix Speaker Fit Guide) or looking up a local shop and giving them a call. Sometimes talking to a professional is the best route because they can help answer other questions you might have.
The most common size in cars are 6.5″ speakers, 6″ x 9″ speakers, and 5″ by 7″ speakers.