They have at least bass and treble controls. Usually 5 to 31 frequency bands, each fixed in frequency. Usually with slide pots to show a graphic representation of the frequency curve. Use compressors on individual instruments, not mix. It will be less audible.
Peaks get through before the compressor actually clamps down.
Faster attack will make for a smoother sound. These are usually there to protect equipment such as radio transmitters or speakers from overloading. As the level of the sound gets below a set point, the signal is turned off, blocking any residual noise that may creep through. If not set properly they can be worse than the noise. A delay by itself has no effect but to delay the signal.