An XPS file represents page layout files that are based on XML Paper Specifications created by Microsoft. This format was developed by Microsoft as replacement of EMF file format and is similar to PDF file format, but uses XML in layout, appearance, and printing information of a document. It is, in fact, more justified to say that XPS is an attempt on PDF, but couldn't get enough popularity as owned by PDF for a number of reasons.
Portable Document Format (PDF) is a type of document created by Adobe back in 1990s. The purpose of this file format was to introduce a standard for representation of documents and other reference material in a format that is independent of application software, hardware as well as Operating System. PDF files can be opened in Adobe Acrobat Reader/Writer as well in most modern browsers like Chrome, Safari, Firefox via extensions/plug-ins.
You can also convert XPS into many other file formats. Please see the complete list below.