VST Compressors are based upon traditional hardware compressors.
Once you understand the major functions, it becomes a little easier.
VST version may become more complex to understand when it involves "knee" etc.
Compressor:
Attack: The rate at which the compressor reacts to the first audio signal present
Release: How long it takes before it lets go of the audio level and allow it to return to its normal volume
Threshold: Think of this as depth. How deep into the volume of the audio will it be applying the effect? Or, how low is the lowest dB level it will compress?
Ratio: This is how much compression you are applying to the signal. It is in a percentage of the original signal compared to the amount of effect applied. a 2:1 ratio of compression is a smaller amount of compression than 5:1 or 10:1. A subtle mastering compressor may be set as low as 1.2:1.
So, when you set up a compressor, you may want to set the attack somewhere around 20 miliseconds, a release around 100 ms, and a threshold of -10dB, ratio of 2:1.
This will react to the signal after it catches the first sample at 20 miliseconds.
It will stay at this result until 100 ms have passed, at which point it will return to normal level.
At a threshold of -10dB, it will compress from the loudest signal level, a max of +0dB, down to -10dB, and anything quieter than -10dB will not be changed by the compressor at all.
It will compress with a ratio of 2:1 which is a fairly noticeable amount of compression but not extreme.
If your audio signal goes all the way to +0dB to begin with, you will hear it compress the loudest 10dB of your audio file with a 2:1 ratio at a fairly quick rate. On drums, this can have a bit of a 'smack' effect. The faster the attack and release, the more of a 'driven' type sound may occur.
Hardware compressors are sought after for their character as much as their usefulness. For instance, an 1176 is an extremely fast compressor and can get fuzzy, driven, and very audible in its effect. A Manley Vari-Mu compressor is known more for a smooth, rich, but cleaner response that allows the music to be unchanged or subtly changed in a pleasant way, without completely changing the character of the music.
Some compressors, like the 1176, are more useful for individual instrument control, and others, like the Vari-Mu, may work better on full mixes. (Yes, the 1176 is a limiter, but it is used for drum compression most of the time, second to vocal and bass).
The typical compressor reduces the overall volume of your file, and brings up the final average or rms volume. This means that it will typically reduce your peaks and thus make the lower signals a higher volume average.
Deep compression is a deeper threshold, and can use very low ratio. This affects the entire signal or most of the signal, down to the quietest levels. Shallow compression can be used just to reduce peaks.
Let's say you want to increase the average drum level. You could use a 10:1 ratio on as little as -1dB threshold on a file at +0dB,and this will compress only the tip of the peaks, but with a lot of effect.
I hope this helps.
Once you find your way around, the search function is a great tool on these forums - there is a fairly massive back catalog of recording info