PUB is the extension used to define files made my Microsoft Publisher. It is also used as the extension for Adobe PageMaker. This article covers the Microsoft product.
This product is a desktop publishing application. It is available separately from Microsoft, but is usually sold as part of the Home Professional Office product. Unlike the other Microsoft Office products the format of this product did not change in Office 2007. However some features would be lost if you opened or saved a 2007 Publisher file in older versions. see Microsoft technical support article.
The product is similar to Microsoft Word except that the emphasis is on page layout and design rather than composition.
No eBook reading software will recognize this format so it must be converted to produce an eBook. Almost no other tool will read this Microsoft format including other Microsoft products. It is designed to support printers, not other formats. Note that a PDF can be created from this format but only if you own Microsoft Publisher. The program can also save pages as images in JPEG format for printing purposes.
The Publisher 2013 version can also send a full publisher document or a page from the document as an HTML format in an email message. It includes email merge so that a mailing list can be used.