It is not the simplest task to find the description of what YouVersion really is. On its front page it looks like an online Bible that users can annotate and interact with. On its "about" page, they talk about their objectives and past, but not what the site can do for you. The support pages are great, but only for existing users. On their blog, you can read the latest entries, including today's entry about the Mashable awards (which was the post I came across elsewhere that led me to the site.) If you go to the very first entry of the blog, all the way back from June 2007, that's where you'll find the clearest description:
YouVersion is a free online Bible that allows users to associate video, audio, images, text, tags, and links to other websites with any verse or series of verses in the Bible. Each piece of contributed content can be labeled as public or private, so the application can be used both as a personal study tool and a public expression of user-generated commentary. In addition to contributing content, Scripture can be organized by assigning user-defined “tags” to any verse of the Bible.