How To Open an EPUB File
The easiest way to open an EPUB file is to double-click on it and let your PC decide which default application should open the file. If no program opens the EPUB file then you probably don't have an application installed that can view and/or edit EPUB files.
Take great care when opening executable file formats received via email or downloaded from websites you're not familiar with. See my List of Executable File Extensionsfor a listing of file extensions to avoid and why.
EPUB files can be opened in most eBook readers, like the B&N Nook and Kobo eReader. EPUB files have to be converted before they're usable on the Amazon Kindle (see How To Convert an EPUB File below).
EPUB files can also be opened on a computer with various free programs including Calibre,Adobe Digital Editions and many more.
Plenty of iPhone and Android apps also exist that allow viewing of EPUB files. There's even a Firefox Add-on, EPUBReader, that allows you to read EPUB files in the browser just like other documents.