Amplifiers, or amps, and another big piece of the car audio puzzle. If you going to be using the amp to power midrange speakers and tweeters, you will want to look for a AB amplifier that is multi channel. If you are planning on powering subwoofers, you will probably want to look for a class D mono amp. You also have to look at the RMS and peak rating on your speakers. If the amplifier going to be use to power subwoofers, you have to know how you are going to wirer the subs. Positive to positive, negative to negative, parallel and series and how this affects ohms and how a drop in ohms affects to output of the amplifiers. You also have to make sure the ohm load matches the ohm of the subwoofer as well. Dont worry, i will soon having wiring diagrams for you to use. Car amplifier reviews will be available in the car audio blog.
Car Amplifier Features and Information, Lets Review

Class: This refers to the way the amplifier operates and is very important to look at. The 2 most popular are going to be class D and class A/B. A/B amplifiers will usually be used on tweeters, coaxials and components. Class D amplifiers are usually for subwoofer amplifiers and can reach efficiencies in the 80%+ range. So this design can be smaller, uses less current and not get as hot as class A/B amplifier

Crossover: A filter is a crossover that only affects one channel, just reducing a range of them.

Separate Gain Controls: This allows the gain of each channel of the amplifier to be set independent.

Stability: The measure of how low of an ohm load an amplifier will be stable at. Any decent amplifier will be 2 ohm stable. The amplifier will normally double its power each time the load is cut in half. This is most useful when running multiple speakers off of a single amplifier.

Distortion: This is often given as T.H.D. Distortion is the measure of how much an amplifier will change its signal from the input signal it receives . The figure can be in the 3% range without being heard, but almost all high quality amplifiers will have a T.H.D. below 0.1%.

Bridgeable: This feature allows a 2 channels to be connected into one channel for more power. This is normally used for powering a subwoofer.
Channels: The more channels an amplifier has, the more options you will for installing speakers.

Efficiency: The ratio of power input (battery) to power the speakers. Lets say you have a 50 watt amplifier with an efficiency of 50% would take in 100 watts of power (battery) and output 50 watts of power to the speakers. The other 50 watts of power is wasted as heat. The higher the efficiency, the better the amplifier. Amplifiers with low efficiency will have a better chance of overheating.

Power Output: The power output of an amplifier will almost always be given in a four ohm load, all channels driven from 20Hz-20kHz. Keep in mind that while the low end amplifiers are exaggerated in their power output, many high end amplifiers are under-rated in their power output.

Pre-amp Inputs: This is a set of RCA jacks that will accept a low level pre-amp signal from the amplifier.

Pre-amp Outputs: This is a set of RCA jacks that pass on a low level pre-amp signal to another amplifier.

Tube Amplifiers: These are not very common and also the very expensive. They are said to produce a a lot better sound in midrange speakers.