Dear Andrew,
Yedda shares its basic mission with Yahoo Answers, Guruza and several other services out there: connecting people looking for knowledge with people who have the knowledge.
But, some say it's all in the details. 
Allow me to cut-and-paste + rephrase from the Yedda about page:
.. Personal:
There are so many topics out there. At Yedda, we carefully select for you, based on your preferences, the questions and answers that are likely to be of interest to you. And, the more you use Yedda, the better we get at under standing your interests. So, go ahead, personalize Yedda to match your interests!
.. Active:
We like to think of each question you ask as if it's the single most important question in the world. After all, it probably is to you. Yedda proactively invites relevant people to answer your question and continues to monitor it to make sure you get the best answers as quickly as possible
.. Smart (well, at least on a good day):
Yedda understands what your question or answer is about, and is smart enough (see disclaimer above ) to find related questions and answers automatically for you even if they're listed under a different topic.
.. Dynamic:
We've built Yedda to be a dynamic, self-evolving system, that adapts itself to the people using it. New topics and associations between topics are created dynamically by the community. Every question you ask or answer contributes to our understanding of what people really care about.
Also, you might want to check out some additional discussions on this topic that took place on Yedda a while ago:
"What is the best place to find knowledge on the net"
"What is Yedda all about?"
"How does Yedda compare to ask.com and ehow.com?"