All an e-book is is an electronic version of a book.
Rather than buying a physical book in a store, you receive the book in digital form.
It's just a file (usually a PDF file) that you download onto your computer, which you can then view on your screen or print it out if you wish. Downloading it onto your computer allows you to keep a copy there and view it whenever you choose.
An e-book still has all the text and photos that a regular book has, but what's really cool about it is that it has even more functionality than a regular book.
Throughout e-books, you can click on links and photos and be taken to websites for more information on whatever you clicked on. This multiplies the value of the e-book exponentially.
E-books are very common online now, and very convenient because you can usually begin reading your e-book moments after you complete your purchase.
To sum up, e-books are usually cheaper than regular books, you have immediate access once you make your purchase and they don't take up any space (at least not on your physical book shelf)!
So if you're new to computers or haven't bought and downloaded an e-book before, don't worry. It's easy to do and you'll get the hang of it. Trust me.