While Nova Spivak has an excellent overview of what Wolfram Alpha is trying to achieve, this will give you an idea:
Where Google is a system for FINDING things that we as a civilization collectively publish, Wolfram Alpha is for COMPUTING answers to questions about what we as a civilization collectively know. It's the next step in the distribution of knowledge and intelligence around the world — a new leap in the intelligence of our collective "Global Brain." And like any big next-step, Wolfram Alpha works in a new way — it computes answers instead of just looking them up.
Doug Lenat has also posted his thoughts on Wolfram Alpha.
The wrapper comes off in two months for everyone to access the service. I can't wait.