Chances are, you have ripped some of your music. More than likely you chose a lossy encoding. Probably it's in MP3 format. Maybe not. Anyway, I have continually refined my ripping strategy. It's optimized for getting the best audio quality on the device I am using. This includes:
I've ripped my cd collection a total of 3 times in my life. The first time I ripped my audio I did so in MP3 format on my Mac. It only supported 32 character file names, and I ripped 160 KBps. The second time I ripped everything in Windows Media Audio Lossless. A few months later my hard drive died and I lost all 200 GB of data. Ouch. This is when I adopted my new strategy.
When it comes to storing and archiving your CDs, I highly recommend the products from DiscSox. I utilize Eurolite4 CD/DVD Case with DiscSox Classic CD storage sleeves. Each CD gets placed in a sleeve and placed alphabetically. Any CD that is ripped gets an Avery Color Coded Label. Because my drives are only 200 GB I only rip albums that I really plan on listening to. When I get my 500 GB drives I'll rip more stuff and it will be easy to find albums that aren't ripped.
So, assuming you are using Windows Media Player 10, set the player with the following options (Tools->Options->Rip Music)
I set the location to D:\Files\Music which is where I have my big drive.
Once you have all your audio ripped I recommend that you run Album Art Fixer to get Album Art for any obscure CDs. This is an awesome app and will also fix other weird problems with the Album Art Meta Data.
Now, you can run the Plus! Audio Converter and convert to another format. If you plan on using an iPod as a portable music player then convert to 160 KBps MP3. If you plan on using a PlaysForSure compatible player then Windows Media Player will automatically convert the WMA lossless to a format that will be suitable for your device. You can optionally select the bitrate manually, or running the Plus! Audio Converter and selecting an appropriate WMA format (I like the VBR variants).
Now for you Mac folks, I don't believe Apple offers this feature in iTunes, and I don't know of any Mac software that can take a lossless AAC file and convert that to anything else. If there is then you would simply follow the steps above except rip to AAC Lossless, and then convert them using whatever software to a lossy compressed AAC format for your iPod.
As a result of this strategy, you will have all your CDs archived away in a nice compact case, full fidelity audio on your pcees, and compressed audio for portable use.