Gizmodo has a smart, interesting, entry-level introduction to various eBook formats. It's probably not a good sign that it's titled "How You're Gonna Get Screwed by Ebook Formats."
The reason just about every ebook uses EPUB is because the vast majority of the publishing industry has decided that EPUB is the industry standard file format for ebooks. It's a free and open standard, based on open specifications. The successor to Open eBook, it's maintained by the International Digital Publishing Forum, which has a pretty lengthy list of members, both of the dead-tree persuasion (HarperCollins and McGraw Hill) and of the technological kind (Adobe and HP). Google's million-book library is all in EPUB too.
While we've only see EPUB on black-and-white e-ink-based readers so far, like Sony's Readers or the B&N Nook, the capabilities of the file format go way "beyond those types of things," says Nick Bogaty, Adobe's senior development manager for digital publishing. Unlike PDF, which is a fixed page, EPUB provides reflowable text, a page layout that can adjust itself to a device's screen-size. With EPUB, content producers can use cascading style sheets, embedded fonts, and yes, embed multimedia files like color images, SVG graphics, interactive elements, even full video—the kind of stuff Steve promised in the iPad keynote.
The public consumes movies, music, and video games in far greater quantities than it does books. Apple's (AAPL) iPad and likeminded devices will make it even easier for consumers to switch off from one thing to the next — or to have it all integrated into one application. But publishers can't help but cheer that there are more book-related apps than gaming apps — and they're hoping to work some magic to fuse the two together.
Fun fact: Earlier this month, Random House hired 15 people to develop video games for them.