What are the various formats for these Project Gutenberg books?
If available, the following formats were downloaded for you:

Audiobook: Several of the Project Gutenberg books also have an audiobook with them, read aloud by a volunteer and compressed in to a variety of audio formats. MP3 is a patented digital audio encoding format using a form of lossy data compression and is the most widely available standard format for recordings. It is a common audio format for consumer audio storage, as well as a de facto standard of digital audio compression for the transfer and playback of music and voice on digital audio players.

EPUB: EPUB (short for electronic publication) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub. EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book. If you have an epub compatible device---like many e-readers on the market today---you can load an epub version on it and read the book.

HTML: Hypertext Markup Language (HTML) is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages. The digital books in this format mean you can click on the “HTML” link under each book, it will popup a new webpage, and the entire book will be available in your web browser.

Kindle: The Amazon Kindle is an e-book reader developed by Amazon.com subsidiary Lab126 which uses wireless connectivity to enable users to shop for, download, browse, and read e-books, newspapers, magazines, blogs, and other digital media. The Kindle hardware devices use an E Ink electronic paper display that shows up to 16 shades of gray, minimizes power use and simulates reading on paper.

Plain Text: Just what it says…it is standard format text that can be displayed within a web browser or any text editing application. The digital books in this format mean you can click on the “Plain Text” link under each book, it will popup a new webpage, and the entire book will be available in your web browser.

Plucker: Plucker is an offline Web and free e-book reader for Palm OS based handheld devices, Windows Mobile (Pocket PC) devices and other PDAs. If you have one of those types of devices, you can load the book on it and read from your device.

QiOO Mobile: PG Mobile, developed by QiOO Interactive, is software that transfers the plain text format provided by Project Gutenberg onto small handset screens – together with all the features known from physical books like turning pages, page numbers and bookmarks. Just download the PG Mobile version of any eBook and read it on your phone: all Project Gutenberg mobile eBooks will soon be available for download as an additional file format in the download section of each Gutenberg title (and each title available here has a QiOO version).