There are three kinds of EQ. Shelving, semi-parametric and parametric.
Shelving EQ affects everything above or below the frequency - something like a low cut filter would be an example of a shelving EQ, with everything below 100Hz (for instance) cut according to the gain (which is essentially a volume control, there's more to it than that, but it helps just to think of it in those terms). The frequencies that are affected are fixed (usually a three-band shelving EQ reads 'High, Mid and Low' or something to that effect).
Semi-parametric EQ allows you to determine the frequency and the gain, so you can 'sweep' and find the specific frequency you need to alter. It does not let you choose the bandwidth.
Parametric EQ allows for bandwidth ('Q' factor), frequency and gain. Bandwidth is essentially the part of the spectrum that is being affected. When you raise the level of a certain frequency when EQ'ing, you will always raise surrounding frequencies as well, so if I were to increase at 600Hz, I would also increase, say, 550Hz and 650Hz to a lesser extent (a gentle curve, like a hill). With a parametric EQ you're able to actually choose the extent of this effect, so you can be much more precise with the frequency you're altering.
http://en.wikipedia.org/wiki/Equalization
The Wiki article explains a lot, but it doesn't go into the full details of it all. You can only really learn how to do it by fiddling around and seeing what happens, but usually you'll find that instruments tend to exhibit similar frequencies, particularly around the 250Hz mark. You'll soon learn what sounds good and how to separate instruments in a mix (if you so desire) just remember that one instrument does not equal one frequency, it equates to many! That is what defines the tone of the instrument along with the envelope it exhibits.
Compression is a huge topic and not one easily covered here. Mostly it is subjective, but it is used just to narrow to dynamic range of instruments or sounds and in doing so, raise or lower the overall volume of that sound. So, if I wanted to make my bass drum hits sound more consistent volume-wise, I would use a compressor. Compressors are used on a wide, wide variety of instruments and often vocals. Most of the music we listen to is heavily compressed. Again, fiddle and see what you end up with. Cubase LE is a good platform for the inexperienced to get that experience.