What is an expander, a gate, or a noise suppressor? What should you do about noise?
An expander and a gate are two versions of one type of processor--basically the opposite of a compressor. Their sensors respond to incoming signal levels just like a compressor does, but they affect the levels in the reverse manner, increasing the dynamic range of your signal rather than decreasing it. The controls, and the meaning/function of those controls, are otherwise the same as the ones on a compressor. The threshold controls when your signal will trigger the effect, and the ratio controls how much the signal is increased upon crossing the threshold.
Typically an expander is used to improve the signal-to-noise ratio of a recording, but it can also be used for special dynamic effects like making a bass line more percussive, or helping vocals pop out in the mix. A gate is a type of expander; the difference is that when your signal level reaches the threshold, a gate goes from "closed" (no signal passes through) to "open" (all of your signal passes through). It's all or nothing, on/off. Some gates offer more advanced control such as ratio or attack/decay, but basic ones are just open or closed. This is a common way of controlling noise and hum from high-gain distortions, single-coil pickups, and other noisy circumstances; when you pluck your note, the gate opens, and when the note dies down the gate closes, keeping quiet until the next note. Of course this means that while you are playing, any noise you had will still be there, at full volume. I don't recommend this unless you are making intentionally choppy-sounding music.
A noise suppressor will either be a gate or a filter that cuts out the high frequency range where noise is usually found (up near 20 KHz)--or sometimes both a gate and a filter. In either case, they work by removing part of your signal, cutting out highs or cutting off the beginning and end of your notes. A few of them do this cutting with less brutality, more elegance, but there is no getting around the fact that they will remove more of your signal than just the noise. Because of this I do not recommend ANY noise suppressor, at any price, unless you have no other option.
Many guitarists will find that with a high-gain amp setup and high-gain pedals there is just no way to prevent getting a wash of noise and feedback; so for them a noise gate may be a necessary and normal part of working with that big distortion sound onstage.
For anyone else who isn't cascading high-gain stages, there is usually a better way to reduce or eliminate noise. Try shielding the cavities of your instrument, or making sure it is grounded properly. Identify inherently noisy pedals, and replace them with non-noisy ones. Try using isolated power supplies for your pedals. Search for ground loops. Check out your gain staging to be sure you have each stage set as clean as possible, often by setting the input gain low and the output level high. This is because input level (gain) is usually an active boost and will add or amplify noise; while output ("master") volume is typically a passive cut from the maximum internal gain, and does not add extra noise. One way or another, it is almost always better to solve the source of noise problems rather than spending money on another piece of equipment just to do a mediocre job of hiding the noise!