A compressor keeps the dynamic range of your material within a "window". It keeps the loud portions from getting too loud, and the softer portions from getting too... The compression is measured as a ratio.
Example: a "2:1" ratio means that, for every 2 decibels (dB) of loudness change at the compressors' input, there will only be a 1 dB change at its' output. A "20:1" ratio will yield only 1 dB of change for every 20 dB of change at the input. The Attack and Release controls vary the way the compressor responds to these volume changes.
You can use the compressor to keep dynamics' inconsistencies (loudness fluctuations) to a minimum, and this can tend to give your vocals, guitar, etc. a more "professional" sound. Sometimes. Sometimes not. We won't go there now. Usually, a compressor is patched in AFTER the mic preamp circuit, and ahead of the recording medium. It is NOT a "mic level" device, but a "line level" one. Typically, you'd patch it into the "insert point" on your mic pre. It will not help you if the mic is allowed to overload the input stage. It can help you out, but ain't no magic pill, either.
IMHO, the ONLY brand to consider in the <$100 price range is a used DBX.
Several models are always on e-Bay: the 166, the 266, even a simple little 163 are ALL better than the Beh^%&%r stuff. They track the program material better, are cleaner and less-muddy-sounding. Check 'em out.

http://en.wikipedia.org/wiki/Audio_compressor

I really like everything that samuraisoundman offered up. Quite knowledgeable. Quite smart. But here I am always recommending one of the cheapest, most prevalent microphone in the world. The venerable Shure SM58 for vocals. Yup, a beautiful microphone for vocals. What? You don't believe that? Ask Bono, Steven Tyler, Michael Jackson and a few others. That's what they use to cut their platinum selling vocals on. No $5,000 vintage U47's with its original tube. A $100 guitar Center special. Just make sure that you roll some of the bass off to compensate for the proximity effect buildup of bass. Since that microphone does not have a switch to do that like the SM 7 which includes low-frequency cut and present boost switches and costs about $200 more.

It's really amazing how good a SM57/58 can sound plugged into a quality preamp like an API, Neve, GML. It ain't your grandma's preamps but the real deal. That's how you get the real sound. Not from the Chinese condenser microphone and the cheap preamps. You get Chinese sound that way which is really fine with hot & sour soup or sweet-and-sour pork, which ain't kosher.

Would you like some balls with your sound? Or is that to go, nowhere?
Ms. Remy Ann David punching out balls with the best

I have more 57s and 58s in my live kit than anything else. I often will try other mics first, but if they don't match the voice well, I can always count on the shures to do a good job.

Are you wanting a performance mic, or one specifically for recording? What are you playing (instrument)? What kind of music?

Thanks very much everyone. Link 555 - that picture on the website helped me understand more than anything else lol. But I will go back and read tomorrow when it's not 2:15 in the morning.

Remy - I'm doin it! I'll buy the 58. Of my songs that I have recorded just with my 57, I almost like them better than the ones with the cheap condensers. I like how dynamics sound, the condensers give a more acoustic but also 'scratchier' sound, if you know what I mean. I will probably still use it for the guitar, but I think the best bet for vocals for me is a dynamic. (at least in this price range) Also how would you go about rolling off the bass? Like doing a simple bass cut after you have recorded?

zemlin - Pretty much for recording. I don't like playing for people really unless I'm drunk. Who wants to show a song to people who might not like it?
I have a song on here called Jersey Shores that's pretty much the same thing as what I do in the rest of my songs. Mostly all acoustic guitar + fruity loops DKFH drums + bass. It's like acoustic rock pretty much. I know I could pay one of you experts to record my stuff but this way I don't have to screw up for you 8 million times and I have the satisfaction of doing it all myself!

Appreciate the help I will post some more stuff soon!

Yup, it's quite funny that the SM58 sounds so good on vocals. A lot of this is largely because of my live recording and live broadcast work. That's the workhorse vocal microphone which I don't have a choice about. I don't need a choice. It is my choice, without even having to make a choice. But it's amazing how close in character it can come to a U87, if you just keep yourself approximately a single fist of distance from the front of the microphone. You use a little bit of low-frequency rolloff or a high pass filter after-the-fact when you are mixing. Of course with the U87's I switch on the low-cut filter when I am tracking vocals. But not always, it depends on how far they are from the microphone. But with what I've described, we have combined U87's and SM58's on numerous recordings and people can never tell the difference! We've even done this with announcers for commercials. I'm not as fond of the Beta 58. In fact it's not one of my favorites. I use those also and the extra bit of high-frequency response can sometimes be good. But I don't find it as smooth as the old SM58.

Of course, the SM58 sounds pretty mediocre with a mediocre preamp. It really shines through an API or Neve. Everybody I've spoken to who knows, has expressed the same thing's.

You're quite correct. Condensers are crispier and for many applications that extra airiness can't be reproduced by other technologies. The inexpensive Chinese and Russian condensers are a bit of a crapshoot. I've heard some that sound awful and others were beautiful bargains. Not as consistent as we would all like. But still a good bang for the buck if you want that condensers sound. And who doesn't? It's all fun.

Funny engineer
Ms. Remy Ann David

That looks like a pretty good deal on the 166XL. I have a couple of those and they have served me well. The one thing we need to be sure of is that your Firepod has insert jacks on atleast 2 inputs. Iknow that their newer stuff does, but some of Presonus' earlier models did NOT...a big disappointment to be sure (especially since they manufacture compressors!). So, assuming yours does have insert jacks, I'd say go for that 166XL.
In all honesty, there is really not much difference in the sound between a 58 and a 57. They use the same cartridge ("guts"), the difference is the windscreen design (metal mesh ball vs. metal mesh/ flat plastic). Very slight differences in tone; most folks here would be able to tell the difference, most lay people would not. You might consider buying a Shure A2WS windscreen (under $10), designed for the 57. It will help you get a more 58-type of performance out of the 57. They are available from Full Compass.
And...screw the graphics you have. It ain't how it looks, it's how it sounds. No two voices are going to "look" the same....

I'm not TOTALLY sure what you mean by 2 inputs? I bought my firepod a few months ago so it is pretty much brand new.

I think I might get the 58 anyways, you can never have too many mics! Not sure yet, but I definitely need a compressor, thanks

Make sure that Input channels ONE and TWO have jacks on the rear panel
for "Send/Return" ("insert points"). I can't make the jacks out on the pic you posted, and the Presonus website doesn't list the Firepod in the discontinued section. I understand that the FP10 has these jacks and that it was formerly called the Firepod. My concern is that some of the Firepods did NOT have these insert points. I want to make sure that your unit DOES have these before you pop $200 for a compressor...