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There's no one universal format, but .epub and .mobi (the Kindle format) cover the vast majority of readers. (I think those cover all the commercially available readers at the moment, except for no-longer-produced readers like the EBookwise that are still being sold.)
If they're available in an open format like RTF or HTML, they can easily be converted into whatever the reader wants; Calibre or other conversion software can convert them.
If you offer your ebooks through Smashwords, it will make them available in several different formats.
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