"e-Publishing", in the RPG community at least, has come to mean "publishing an RPG book in PDF format and selling it online". That may or may not be what you're asking about.
More generally, "e-Publishing" can mean any kind of electronic publishing.
What's your question, exactly? While nobody's likely to have time to define, explain and describe the entire concept of e-Publishing to you, including software, retail outlets, payment processing, marketing, costs and so on (well, if I ever do it'll be an entire book, and I'll sell it!), if you could narrow down the question, I'm sure people will be happy to answer you.
So what do you want to know?