Minifying CSS takes the beautified, well formed CSS code that you have written and removes the spacing, indentation, newlines, and comments. These elements are not required for CSS to be used successfully. It also makes the CSS more difficult to read. The best practice of many developers is to maintain a beautified version, and when rolling out their project will run the styles through a minification program. They will also combine their many style files into one file.
The CSS Compressor greatly reduces the size of standard CSS code, stripping out unnecessary characters and implementing CSS shortcuts, allowing you to minimize the size of CSS file downloads on your web site. The server processes it on-the-fly and returns the output to you without storing anything. So if you use this tool to compress a super-secret bit of CSS code (if there is such a thing) it is not stored or read by me.