The first, and most important thing to think about when you are buying an e-reader is what type of screen you want the e-reader that you choose to have. Are you going to be using the e-reader mostly while you are in the house or mostly while you are outdoors? Are you going to want to watch movies and television shows on the e-reader as well as simply reading books on it? How about full color magazines? Basically, there are two types of e-reader screens. The first is the black and white screen that is perfect for using outdoors. While it does not support color, it is glare free and is less taxing on the battery than a color screen and therefore allows for the user to be able to read more outside. The color screen, however, makes color photographs like those found in textbooks and magazines look better while allowing the e-reader to be used for things besides reading, like catching up on a television show or watching a movie.
Next, consider the formats that the e-reader supports. This can be confusing, since for most people a discussion about formats may as well be in Greek. Fortunately, there are only a few different formats to be aware of, and if you have not bought any e-books or had an e-reader than this does not really apply to you. First, there is the PDF format, which most documents use and that is supported by most e-readers. Then, there is the MobiPocket format, which is supported by most e-readers as well and is the format that all Kindle books come in. Finally, there is the ePub format which is not supported by the Kindle and which can be protected to not allow for converting into MobiPocket.
There are many e-readers available in the market today, and you can use them for a wide variety of things. Once you consider the screen type and the formats, though, hopefully your search narrows.