FireBros. said:
A compressor is sort of like an automatic volume control, that can be used to smooth out your playing, or accent certain parts of a note, especially the decay portion. People often use compressors to give a sense of more sustain without having to use a lot of distortion.
A noise suppressor, also called a gate, or expander, is also an automatic volume control, but it is used to turn down the signal between notes in order to lessen or mute any unwanted noise like hiss, or hum.
To put it simply, a compressor turns your guitar signal down slightly When it hears a loud note, and a noise suppressor turns it down when it doesn't hear a note.
Being automatic, neither of them work perfectly, and some very cool sounds can be had by intentionaly setting them incorrectly. For example setting a comp to get rid of all the attack of a note but making it sustain for days, or setting a suppressor to get rid of all sustain for a very jerky on/off kind of sound.