From https://fileinfo.com/extension/epub 

"An EPUB file is a digital ebook saved in the EPUB format, an open XML-based format for digital books and publications. It provides a standard digital publication format for publishers and consumers. EPUB files can be viewed with supporting software programs and hardware devices, such as the Sony Reader and Barnes & Noble Nook."

It's an old format that never quite became the standard its supporters would like it to be. iBooks, GoogleBooks and Project Gutenberg support EPUB but it does not run on Kindle readers without conversion, for example. It is an xml based protocol as opposed to pdf. The Wiki article gives a good overview:

https://en.wikipedia.org/wiki/EPUB

I suppose it's good that Finale supports it. I wonder, who uses it for what?

Great...thanks guys for your advice and comments.

Mike (from Australia)