File Compression

Anyone know how programs like PKZip and WinZip create their compressed files?

I figure it is something about comparing like bytes and removing any bytes that are repeated....

If anyone knows of a good site or has an example or can just explain it, that would be great.

Take a look at this
www.vbaccelerator.com/codelib/zip/zip.htm

Thanks, but I am actually interested in the code that zips the file.... I am looking for the actual byte format the file is in before and after zipping/compressing.