For those who do not know what compression is a little background is in order. First of all, what compression does is clips or minimizes the high end and low end to allow for the overall recording to stay closer to the maximum output redline for a higher proportion of the time. Let's assume a normal audio frequency is on a scale from 0 to 100. With compression you essentially compress this entire range. For example you might compress it down to the range from 30 to 70 on the previous scale. Then you slide the whole thing so that it peaks at 100. You have a perceive sound that never goes below 60% of the maximum volume. Pretty cool if you want loud. Bad if you want dynamic range.
Many studio engineers will use a small amount of compression for the CD mix to allow for a louder sound. Some artists like this some don't. For the last several years radio stations have been increasing the compression ratios that they use in an effort to make the music "louder".
Radio stations seem to have decided to take this already questionable technology and take it way overboard. I haven't listened to more than a song here and there on the radio in a couple of years but this afternoon none of the CDs I had in my car really appeared to me so I flipped over to the radio and heard the beginning of Steve Earl's Copperhead Road. Life is good. As I listened to the song it just didn't sound like I remembered it but I could not put my finger on it. Then GNR's Welcome to the Jungle came on and it hit me. The highs were gone. As I listened a little closer I realized that the lows were either wiped out or clipped. That's when it hit me the bastards were over compressing the music to an extreme I haven't heard before. I had to change the channel to another radio station that had inferior music but an engineer who was reasonable.
The worst part is that I'm about as far from a music aficionado as you can get. I like metal and punk. I've listened to hundreds or thousands of hours or live audience recordings. I listen to and enjoy muddy recording done in garages and I find excessive compression unlistenable. It just takes perfectly good studio engineering and throws it out the window to create a "loud" sound.
Please radio stations, stop the compression madness. I promise you if I want the music louder I'll turn the radio up. If that's not sufficient I'll buy a louder sound system.
the description above is pretty rough. For a more thorough (and correct) description see the Wikipedia article.