Well, usually .azw for books, but it can also read .txt files and .MOBI files.
From Amazon Kindle User's Manual:
Supported Formats for Conversion
In addition to the file formats listed above, you can also convert other personal documents to read on your Kindle. The supported file formats are listed below:
• Microsoft Word (.DOC) • Structured HTML (.HTML, .HTM) • JPEG (.JPEG, .JPG) • GIF (.GIF) • PNG (.PNG) • BMP (.BMP) • Compressed ZIP (.ZIP)
In addition to the above the Kindle can also display PDF formated documents.
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