Designer Pro X is the flagship application which has all the features of the much cheaper but more targetted applications for specific uses.

Xara Xtreme is now known as Photo & Graphic Designer
Xara Xtreme Pro is now the Designer Pro X

Web Designer is a web centric version of the Designer family
Page and Layer Designer is a text/document layout focussed version of the designer family

You can see the comparisons at
► http://www.xara.com/designer-pro/compare/

In short, Designer Pro X is the flagship product which the others are subsets of.