Well, the about page provides a helpful not-quite-Venn diagram that points out that the site is really a combination of wiki, digg/reddit, blog and forum:
To answer your question, all you have to do is look at the three remaining areas. I'll answer all four, so that this answer can be used to answer "how is Stack Overflow different/better than just [some subset of the four things]?"
Digg, Reddit and similar services allow users to vote on content. It puts the wisdom of the Internet crowd to work for you in judging which posts are worth listening to and which are... well, not.
Blogs tend to contain longer, more essay-like and/or comprehensive posts than other media on the Internet. But perhaps more importantly, bloggers can develop impressive reputations over time, based on the content they post. Conversely, if you see a post by Eric Lippert, you know it's probably worth reading, even if you didn't know he had a blog (or in our case, a Stack Overflow account) before.
Forums are about getting many people together, and treating them roughly as equals. They tend to focus on one topic or a group of related topics.
Wikis are editable by many people, and contents of wikis are generally shared under some sort of non-restrictive license. User-contributed content on Stack Overflow (and all Stack Exchange sites) is licensed under CC BY-SA.