"Glue" compression

I've heard of the term "glue" compression before, and I understand the concept of how it makes things more cohesive somehow, but I don't know how to do it. I only use compression either to even out vocals or as a final mixdown step to make everything a bit louder. When I look at the presets that come with compression plug-ins, they all seem so subtle -- too subtle to bother with. 
 
Do I need to bother learning "glue" compression?

Sometimes the "glue" is simply running the audio through a compressor without doing anything. What you're looking for is "character" of the compressor, not to simply change the audio drastically. Some examples of these types are:
 
Sonar:
LA-2A
4K Type
 
Waves:
CLA-2A
Puigchild 670
API 2500
SSL 4000 Buss Compressor
Kramer PIE
 
These are just some examples. Most, if not all of them are "modeled" compressors. In many cases, a compressor with a slow release, low threshold, but VERY low ratio, such as 1:1.2, for example, just enough to impart slight characteristics to the mix without being noticed, is what to shoot for. In my experience, sometimes running a mix through the PuigChild or the LA-2A compressors, those with no control over attack times and merely boosting the input with a very low threshold is all I need. Enable and bypass the compressor and see if the audio is more musical.

Many times, it's more subjective than a significant change, but what the glue does is make the song more musical on larger systems, and perhaps even soften some edges in a harsh recording, lowering long-term ear-fatigue.
 

Without all the technical jibber jabber... it's bus compression.
 
Create a bus.
 
Send a bunch of tracks to it.
 
Put a compressor on the bus.
 
This compresses or "glues" the tracks being fed to the bus/compressor together.
 
Examples...
 
Drums: You send all your individual drum tracks into on bus. You put a compressor on to smash them altogether... or "glue" them together.
 
Master: You send ALL the tracks in your project to a bus. You put a compressor on it to smash them all together... or "glue" them together.
 
That's all it means. The act of compression just makes separate tracks/sounds sound more "together" and "bunched up"... so the term "glue" is used to describe it.
 
One company went out of their way to market a compressor called "glue" which is just silly and confusing because any compressor can act as "glue". The term itself is kind of silly too.
 
No big mystery really.

One tool I'm finding nice as kind of an alternatively flavor of "glue" is the Cakewalk PC Tape Sim. I guess the tape emulator action includes a bit of compression to it... IDK... but it does seem to provide some interesting cohesive gel that's a little different than a straight compressor.
 
CA2A/PC2A is golden for this too (which was mentioned earlier I think).
 
Another thing I find works nicely is two stage compression so both compressors just do a touch of compression each and it brings everything up nice and evenly. No pumping or whatever (which of course don't want) but you still get things blending together nicely.
 
So it's a BIT more than just cramming a comp on a bus... but almost. More like light/smooth/even compression. Not all pumpy and stabby.
 
I should have said that.

I nearly always have the PC-4K Compressor on my Master bus for this very reason.  It is perfect for "Glue" compression, as was the hardware on which it was modeled (an SSL Bus Compressor).
 
I leave the ratio a 2:1, and aim for < 4 dB gain reduction on the loudest peaks.
 
I adjust the Make Up gain so that the signal sounds at a similar level with or without the compressor.
 
I may adjust the Attack and Release times dependent on the material, although the default settings are a good starting point.
 
As to what/why I think "Glue" compression is.. maybe tomorrow, after a night's sleep..
 
 
 

I'd add the one point that no one's mentioned yet, but which you've probably already suspected: glue compression is entirely optional.
 
Back in tape days, it wasn't optional. It was built into the tape. Many people had a hard time switching over to an all-digital environment, and a big part of that was they were missing the tape effect. Tape was usually driven into saturation, which applies compression - "glue". And that makes the mixer's job easier. Tape sims and glue compression are an attempt to recreate that effect.
 
But it's not necessary. You can choose to embrace the clarity of digital audio and provide your own "glue" through level automation, track compression and EQ.
 
We can do things now that were only a dream 40 years ago, such as applying dynamics control to every track (yes, one console manufacturer did provide primitive compressors on every channel, but most of us couldn't afford those). Back then, you might only own one pair of nice compressors, so you had to carefully choose which track or bus to use them on. Now you can have any number of compressors. Back then, putting a fully-parametric equalizer on every track was not an option, now it is. Back then, volume automation was limited to imprecise broad strokes; now you can automate the tiniest details with millisecond precision. 
 
I do use bus compression, sometimes. But when I do, it's an admission that I've not done a good job on the mix and need a crutch. For those of you who routinely use bus compression, try this experiment: pull up an old project (old enough to hear with fresh ears), remove the comp and volume-match the original. Then A/B both versions of your mix and note how the uncompressed mix pops to life.