You're misguided in several areas here.
For starters, what type of EQ are you using, and what kind of compressor are you using?
What is your DAW platform?

DonnyThompson, post: 443037, member: 46114 wrote: You're misguided in several areas here.
For starters, what type of EQ are you using, and what kind of compressor are you using?
What is your DAW platform?

Im using CuBase, and lets take the built in compressor and 4band EQ as an example.

EQ is a tonal filter, as you mentioned, except that it's not just used to filter out frequencies, it can also be used to add or to boost frequencies, too.

Filtering out is known as "reductive" or "subtractive" Equalization, while filtering in -which is boosting or adding certain frequencies or frequency ranges is known as "additive" or "boost" EQ. Both can be useful, depending on what you want to do.

Compression is Gain Reduction ( GR), and can be used in slight amounts, or, in heavy amounts. Its primary purpose is to keep a track's volume at a consistent level; although using it in heavier amounts for "effect" can also help to bring tracks forward so that they stand out in the mix.

While EQ and GR are not the same things, you can use them together. A popular way to use a compressor is to side-chain it to an EQ so that the compressor only reduces the gain of certain frequencies.

Some compressors will allow you to set which frequencies - or frequency ranges - you want to compress and which ones you don't. The width of the frequency range is known as the "Q". Higher Q values will narrow the band you are working with, so that you're only effecting a limited range. Lower Q values will cover more areas above and below the primary frequency you are wanting to effect.

To keep this simple, the best first step you can take - assuming you have a well-recorded guitar track in both tone and performance - is to add a Hi-Pass Filter ( HPF) to the EQ first, so that you are rolling off, or cutting frequencies that don't exist in the track's natural frequency range. For example, there's very little of 100Hz energy on an electric guitar; so allowing low frequencies like that to "pass" through, is doing nothing of benefit for the guitar, and allowing those frequencies into the mix could instead be picking up background noises that reside in low frequencies that you don't want - traffic, heating or cooling systems, construction a block away, etc. Filtering out those lower frequencies that don't do anything to help the guitar stand out in a mix allows the track to be more controllable in the mix, because you're not having to deal with unwanted noises that do reside in those lower ranges. A Low-Pass Filter ( LPF) works the exact same way for higher frequencies. There's no point in adding 10k to a 4 string electric bass guitar track, because there' nothing up there tin the natural sound of the bass guitar o work with. Cutting out those unwanted and unneeded frequencies help you to define the bass more in the mix, without having other "clutter" frequencies to get in the way.

There are several schools of thought on which one should come first ( in what order). Some use a compressor first, to tame the overall peaks of the track, while others prefer to use am EQ first, in order to filter-out frequencies that don't add any sonic value, to the instrument you are working with...but that could still be strong enough to cause the compressor 's detection circuit to kick in anyway ( referred to as a false trigger).

Before we go further, does this help?

Yes, I already know some of this but I really appreciate what you wrote.

Could you also explain what 'gain' actually is. I know that the volume gets louder but not the technical reason.

Please, go on, I find this very interesting and educating!

Compression can "undo" eq to some degree, but it's not a strong effect since a real musical signal has the various frequencies happening at once. If you reduce one frequency range then compress you'll still hear the relationship, the relative difference, between the range that's cut and the other frequencies.

Gain is a change in level. It can be positive or negative. Gain reduction is negative gain.

I've found reading through our audio terms to very beneficial.
Maybe grab a coffee during some down time and look through here: http://recording.org/forums/audio-terms.48/

About the switching around. You can compress the signal before or after an EQ.
It will give different results depending on the source.
ex 1 : if a signal has a lot of bass it will make the compressor work harder. So if you EQ out the bass before the compressor, it will sound more natural.
ex 2 : you want to add a lot of bass without affecting the compressor work, put the EQ after..

There is a ton of other exemples but your first goal should be to master them individually and then study the effect one has on the other ;)