The first link is a good one about compressors the other link covers other audio topics with some info on gating uses.
Heres a quick overview:
compressors basicaly take an audio signal in an electrical form and make it smaller then the input signal. Often used on CD's so that the signal level is kept fairly even between tracks. It can also help minimise distortion that occurs if to large a signal is let through to the amplifiers etc.
Gates do what any gate does. It lets things through or stops them. In this case this an audio signal, once again in electrical form.
The gate can be set so that a signal must be above a certain level before it is allowed to move further in the audio chain. It can be also set that once it has been opened by a signal level high enough it will close again once the signal falls below a certain level. These levels can be varied on the gate.
Gates are often used on drum kits. For example if you gate the mike on the kick drum it will only let the signal from that mike pass once the level is high enough. It helps minimise pickup from other sound sources getting into the mike. Also the signal level at which the signal level coming from the kick drum mike falls low enough that the gate turns off the signal from the mike. This means instead of the drum sound keeping going until it finaly stops the gate cuts it off crisply. You can also use a gate on the snare drums,cymbals etc. Once again the signal level to turn the gates can be set that only when the drum,cymbal for that particular mike is struck the gate turns on. It helps minimise pickup from other sound sources and gives cleaner cutoff.
There is plenty of good info on this subject on the Web if you google for it.
I'll take a crack at explaining the knobs every compressor and gate should have. Compressors are used to turn the loudest parts of a signal down in order to allow the signal as a whole to be turned up. The threshold knob will be on every compressor. This knob sets the volume at which the compressor will start doing something. When the volume is as loud or louder than the threshold is set for the compressor does something, otherwise it is passive. Often compressors will have an attack knob that sets how long the signal must be over the threshold before it kicks in. Once signal is over the threshold the ratio controls how much volume is removed. A 4:1 ratio means for every 2dB over the threshold 1 dB of volume will be removed. A compressor with a 1:infinity ratio is known as a limiter. It will not let volume exceed the threshold. The release knob sets how long it takes for gain reduction (or compression) to stop and the compressor to be once again passive. A lot have an output knob to give you make up gain for the gain lost.
Gates have similar knobs. Once again every gate should have a threshold knob. The threshold sets the volume it becomes active again. With gates however it is different. When volume is below the threshold it does something and when it is above it is passive. When volume is below the threshold the gate turns the sound off. When the volume hits the threshold for the attack time the gate opens up and gain reduction turns off. The hold time sets how long the gate stays open for once volume gets back below the threshold. The release sets the time it takes for the signal to turn back off after the hold time.
Now to illustrate this a compressor with a -12dB threshold with a .2ms attack time a 4:1 ratio and a 200ms release with a makeup gain of 2dB would do the following.
Nothing while the volume is under -12dB. Once the volume hits -12dB it does nothing. Once the volume has hit -12dB for .2ms the compressor begins to activate. Should the volume coming in be -8dB the output would be -9dB as it is removing 1 dB for every 4 over the threshold. It will continue compressing in varying amounts while over the threshold, then when the volume goes under the threshold the compressor continues compressing for 200ms then it would once again be passive. The entire time if the audio is above or below the threshold 2dB will be added to the signal.
To illustrate a gate lets take a gate with a -24dB threshold, .2ms attack again, with a 1.5s hold time and a 800ms release.
The gate would stop all sound going into it. If the volume is above -24dB it does not let any sound through it unless it is at -24dB or more .2ms or longer. The gate will deactivate allowing all sound to go through as it was coming in until 1.5 seconds after sound was under the threshold again. It would then take 800ms to close again and no sound would be allowed through.
For applications, gates are most often used on the tom mics of a drum kit. The threshold is set high enough that no sound goes through the toms, but low enough for the gates to open up when the toms are hit.
Compressors are more frequently used. I have used compressors on kick and snare drums, bass guitars, acoustic guitars, vocals, electric guitars, keyboards, and the main mix. Compressors are your friends when set properly, but the key with setting a compressor is that you should not be able to hear when it is on. Have you hard the volume of a song change on the radio? That is a compressor, and it is annoying to me personally. Try playing around with both if you have a chance to. If you have someone who starts screaming into the mic a lot, or a musician that plays quiet then loud they are nice to have.
Sorry the post was so long, but there is a lot to cover. I think that is most of what you can learn from text though, reading is not a substitute for playing around with them.
Might have been said already, but just to to re-emphasize; a compressor squishes the entire signal, not just what is above the threshold. So in the above scenario, even the signal that's under -12dB will be compressed. It might not make a difference used as an insert on 1 channel's insert, but if you are compressing the main mix, groups, or auxes, just beware of it.
Just thought I'd clarify. I know people are hazy on this area.