alrite i'll start with the most common one.
do compressors tighten up your sound?[/b]
The simple answer to that question is yes.
Now, take a cup of nice hot or ice milo or your favourite drink, get comfy on your cushy chair and take a deep breath....Ahhh... Ready!? No la not so long la Hehehehe.
Ok, so the shorter version of a long answer is that compression creates that "tightened" effect by squashing the initial attack of the note and raising the fading signal. Depending on how one sets the "sustain" knob of eg. CS3 the dynamics range (level difference between softest and loudest) will be narrower. Also as the compressor keeps trying to raise the fading note it creates that sutain effect.
Compression when tweaked well works to the guitarists' advantage in a group setting because it keeps your sound level more stable as in your signal will be more consistent and the soundman will love you for it because he doesn't need to keep pushing the mixing board faders up and down for you within song because your signal previously without dynamic consistency was either too soft or too loud.
So in the way described above yes compression will tighten your sound.
Now if the compression effect was applied more than liberal 2 things will hapen to your disadvantage;
1. Your signal will be too squashed and lifeless as in there's no dynamic change where everything sounds like a flatline without peaks and valleys in your phrasing, your chordal and soloing.
2. Because the compreesor is set too high it keeps trying to raise all the soft notes including the "silence" betwwen notes and musical passges. This will result with undesirable noise. This is a no no of course.
Hope this helps ya.