A person who opens a CLS - LaTeX Document Class File will not have a clue what it says. This is because the file is written in LaTeX, a document markup language that is generally used for typesetting academic documents. This particular file type can be imported into a .Tex document as and when needed and is used to store various types of templates.
The average person is not likely to encounter this particular file type, but it is commonly used by groups of academics who write for a particular journal or paper. This file type ensures that the documents written by various contributors all have the same basic template.