What does "compression" like?

Data compression algorithms for digital files are entirely different than compressor/limiters techniques used in audio processing. For nearly all pop/rock recording styles a little compression makes the recording sound better.

Simply, if you notice on different recordings, the volume knob for one recording, you barely need to turn it on and the music is very loud. Other recordings you can turn the volume knob half way before it gets loud. The one that you barely have to turn the volume on is what we would call very compressed. Especially if you are familiar with a song live, if you know the beginning is soft but on a recording it is loud, then the volume has been raised for the beginning. Basically the whole song is at one volume level and any changes in volume during the song (dynamics) are gone.

Like Wildoats said. Plus if the singer is the same volume as the drums and guitar.

I've seen a few user reviews on Amazon where the reviewer says that something's wrong with the CD since he needs to keep changing the volume during the song.
I always feel like making a comment, but it might get ugly.

Thanks a lot for all the educational replies, folks! Now I understand. And what I
was asking about was dynamic range compression, not file size
("lossy") compression of digital files.

Again, very helpful!

Rebbi,

Just as an aside, not all file compression technology is lossy.

FOr example, FLAC files can be compressed without losing any digital data, although compressed file sizes will be larger than lossy compression, like MP3, but smaller than uncompressed.

I just converted my music library from uncompressed .wav files to lossless compressed FLAC. Overall size went from ~ 900+ Gb to about 500Gb, which, based on my personal professional experience in the past developing computer image data compression algorithms, I would expect to be typical lossless compression ratios for most digital media content.