Each of these machines uses a different file type. The most common are epub, mobi (azw), and pdf. There's a really good explanation of the different file types in Wikipedia. Even thought each reader uses different files types, most books are available in all the file types so you should have no problem finding books for the reader you pick.
All the main reader types also have applications that will let you read the same books on tablets (like the iPad or Xoom), on your computer, or on your phone. You can start a book on one device and finish it on another. I have a Kindle, I have the Kindle app on my Xoom and on my cell phone and on my computer and on mom's work computer. Mostly we read on the Kindle, but sometimes mom leaves it home. Then she reads on her phone. All the devices can sync up to each other! I think Nook does the same thing.
Not only can you coordinate all the devices, but you can highlight stuff and make notes. And if you're reading a book borrowed from a friend or the library, you still can make notes & read the same book on multiple devices. you get to keep your notes even after you return the book.
Borrow books from friends and from the library? That's right! Both Kindle and Nook let you do that. I don't know about the Kobo & Sony, but I bet they do, too. Not all books. That's controlled by the publisher of the book as to whether or not they turn that on for any particular book. Check with your library as to which ebook program they're in and what readers they support.
Some eBook readers also let you listen to audiobooks or read the book to you (text to speech). Text to speech is another of those things controlled by the publisher, so it's not available on every book.
Another advantage of eReaders is the amount of free books available. Authors and publishers give away free books trying to get new readers. Amazon and B&N both have of listings of hundreds of freebies. Lots of the books that are in public domain are available for free. You could read for the rest of your life without spending a penny on books!
There are other advantages to eBooks, too. They don't take up space in your house. eReaders are small and light enough to carry around. In fact, you can carry your entire library in your pocket! Having trouble finding large print books? No problem! Just change the font size on your eReader. You can make the fonts very large if that's what you need. Mom has trouble with her hands and it hurts to read a regular book, but she can hold her Kindle just fine. The Kindle has made reading so much easier for her that she's gone from reading just a few books a year before the Kindle to reading a book every week or two like she used to before her hands went bad.
Some eReaders have eInk screens. These aren't back-lit so you need light to read them. They use power only when turning the page, are easy to see out in sunlight, and the battery lasts for weeks between charges. It's the closest to reading a real book. Some have LCD screens. They're back-lit and some people find that harder on their eyes for long reading sessions. They need to be charged more often. They have the advantage of being more like tablets so they do lots of things besides read books, and are in color. These are things to think about when you're picking a reader.
I could go on and on and on, but this should be enough to get you started. I'll be happy to answer questions if you have any. Just leave your questions in a comment. I'm going to leave you with a list of places to get free books besides Amazon and B & N. If you know of any other sources, let me know so I can add them to the list.
This article originally appeared at Adventures of a Suburban Kitty.