Yikes....that's a very general question.
A signal processor (the generic name for an EQ or Crossover) simply does what then name suggests: Processes the signal from your head unit.
An EQ basically "forms" the signal that your source unit puts out. By using one of these you can control where frequencies peak and generally fine tune the output to your liking.
A crossover keeps unwanted frequencies out of certain "channels" of your audio system. IE, the lows go to the subs, but not the mids and highs, the highs go to the tweeters, but not the lows and so on and so forth. This helps a lot and keeps out-of-range frequencies from damaging your equiptment.
Both EQs and Crossovers go "between" the source unit and amp, and are usually connected via "low level" or RCA inputs.
"There is no 'I' in Tag Team"
Project 'rex: Coming Soon to An Eardrum Near You