Advice about eBook Readers for Mac/PC
ePub is a digital publishing format, but it's not as straightforward as you might hope, in that not all ePub files are the same.  There are 3 main types of ePub in the wild today:


Straight ePub, any ePub reader should be able to open these
ePub with Digital Rights Management protection:

Adobe Digital Editions
Apple iBooks from the iBooks store



The protection mechanisms are portable to an extent, you can see the Digital Editions protection on other formats other than ePub.  The main takeaway is that to read the file, you need a compatible reader.  For the non-protected files, that is straightforward enough.  Pretty much anything can read them.  For the protected ones, it's more difficuly.  Adobe protection is compatible with a small number of desktop apps for a bit of extra portability, but iBooks ones can only be opened in iBooks in iOS, full stop.

It's a pain, and like the MP3 fuss we are starting to see people dropping DRM entirely from their ebooks.