A noise gate is a device which cuts the signal when it goes below a certain amplitude. That is, when you don't play your guitar, the noise gate will make it totally quiet, for example. You can adjust the level at which it kills the signal.
Noise suppressors are devices which attempt to filter out noise from your signal, leaving back the tone. Unfortunatly, it is impossible to figure out what part of a signal that is noise and what part that is tone, so killing the noise will also affect the tone (if only slightly for the best suppressors).
I would suggest spending your money on getting your guitar shielded properly if you have noise problems.
Someone else can probably suggest you something if you wanna go with a pedal
