If you store data in a format that's not future proof, using software that's not open source, then you don't really get to complain when your data disappears. Especially if you're not storing the files locally and making regular backups.
They use a directory tree, with one directory per note. Each note has HTML file, all supporting files, an "enml" version of the HTML file (i.e. a HTML-like XML document), and XML documents containing character recognition data.
The local indexes are in lucene format. (Specifically clucene.)
You will also find a sqlite3 database with additional metadata.
If you store anything you care about in Evernote, I'd highly recommend backing this up. On OSX it will be in within one of these two directories, depending on which version you're running:
~/Library/Application Support/Evernote
or ~/Library/Containers/com.evernote.Evernote/Data/Library/Application Support/Evernote