Amplifiers: A multi-channel amp will power your front and rear speakers, or front speakers plus a subwoofer or two. Most include crossovers to adapt the amp to your needs.
 

Equalizers: Multi-band equalizers allow you to boost the frequencies your car absorbs and reduce the ones your vehicle exaggerates a must for the serious car audiophile.
 

Crossovers: Crossovers contain filters that divide your music into frequency ranges (high, low, and midrange), then direct signals to appropriate amps or speakers.