It seems that every technology creates a standards war: VHS/Beta, Windows/Mac -- even technology that's been around as long as the automobile still hasn't attained universal agreement about which is the proper side of the road for driving.
The ebook is no different: there are many competing formats and (putative) standards, and every player in the market seems to have settled on a different one. This becomes even more confusing if you have more than one device which can display ebooks (e.g., an Amazon Kindle and a smart phone) or a device that can display more than one format (e.g., an iPad which can run the Kindle app as well as using Apple's own native ebook format).
Calibre, a free and open source package, can help. Calibre organizes your ebooks into a searchable database, will display most common formats, and will even convert one format to another. It will also sync your book collection to your device, and has a nifty packaging feature that lets you convert web feeds into ebooks for later reading on your mobile device.