ZIPX (.zipx) is a file extension for a compressed file that can be created/opened with WinZip (they first introduced the format) or Bitzipper. A Zipx file is basically a zip file but created using advanced compression methods, supported in the latest version of WinZip and also by other zip utilities (Bitzipper is the latest one to add ZIPX support). Because of these new compression methods introduced by WinZip, zipx is practically the smallest zip files up to date.
With the latest version of WinZip you will create .zipx files by default, but if you want you can still choose to create "Legacy Zip 2.0" compressed files that are more common and for which there are more utilities available.
ZIPX was introduced as a default compression method starting with WinZip 14 which was released in May 2009. To be more specific, the .zipx file is a .zip file that uses one of the following compression methods based on the content to be archived: PPMd, WavPack, LZMA, or bzip2. This results in a smaller size for the archive, since images will be compressed using an image compression algorithm, multimedia files will be compressed using a more adequate method and so on.