The Kindle is actually a software platform that runs on devices that go by the same name. The Kindle family of products represents arguably the biggest e-reader in the market, with the most current iteration offering Wi-Fi and 3G support. Amazon also offers sales via its Kindle app for competitor devices, and is probably the most heavily advertised and publicized of the current e-readers.
DRM
Amazon's Kindle comes with its own file type for digital books, AZW. While users can upload files from other formats from a computer by USB or by emailing it to Amazon, it is technically against the Kindle terms of use to transfer AZW e-books to other users or other devices.
The Kindle supports Mobipocket files (MOBI) types, and TXT files, but other formats must be converted to AZW through the e-mail Amazon provides (there's a 15 cent fee to convert directly into the Kindle, or its free if it is sent to a personal e-mail). Epub files, quickly becoming the standard format across all devices, is not supported on Kindle, but can be converted into an acceptable, readable format by Kindle, using software like Calibre.
Books downloaded from Amazon can be held to a limited number of devices at the same time, depending on the title and licensing agreements with the particular publisher. Anywhere from one to six copies are allowed at one time; users can unregister some devices in order to add new ones.
Marketshare
Amazon is leading the market in e-reader sales with approximately 47% of the market in a recent survey. Like other e-book sellers, Amazon isn't sharing its sales figures for e-books, but did boast that for every 100 hardback books the online retailer was selling, 143 digital copies were being sold.
Conclusion/Review
Amazon's Kindle is widely popular for a number of reasons, but is surprisingly inflexible when it comes to DRM. The AZW format is not helping the cluttered scene of other formats, but because of the platform's popularity, is a real contender to be "the one format to rule them all." Worse, the lack of support for ePub is baffling, especially since almost every competitor's device allows it. Still, the Kindle app for other devices is robust and freely available to Kindle customers. Of course, the workarounds for Kindle's lockdown on its own device look promising, as do some content providers of public domain titles supporting the AZW format (like Project Gutenberg or the World Public Library).
As the current market leader, Amazon still has room to improve its sales by unrestricting the digital books it sells through its storefront. Whether the demand for it appears for the Kindle is another story.
