Book covers displayed on product pages in Amazon’s Kindle store are emblazoned with a special logo: the words “kindle edition” appear below the book cover, and a small image of a Kindle reader is superimposed onto bottom-right corner of the cover itself. This superimposed image obscures part of your cover, so when designing book covers that might some day be used for Kindle editions (including covers that are originally intended for use with paper editions or ePubs) you might want to keep important graphical elements away from this bottom-right area. Alternatively, you might prefer to create custom Kindle covers that take this issue into account.
You can probably judge the obscured area by eye, but if you want to set up guides showing its extent, you could always find a suitable cover image on the Kindle store — one that has the same aspect ratio as your book cover — and then crop it appropriately, enlarge it to match your cover, and paste it into a layer within Photoshop or whatever equivalent program you use. The pasted cover then becomes a template from which you can position guides to show exactly where the “unsafe” area is.
We generally create ebook covers at 600×800 pixels, and the unsafe area is then approximately 82×146 pixels.