It's pretty well known that Amazon Web Services' EC2 and S3 initiatives have taken off and are gaining users not just in startup land but in Corporate America as well. Amazon provides a compute and storage cloud, and the rush of companies big (e.g., Google) and small (e.g., Nirvanix) beginning to compete with Amazon in providing clouds has spawned a term and a movement - Cloud Computing.
I've been involved with precursors to cloud computing (Utility Computing, Grid Computing, Application Service Providers - ASPs) from my days at Corio, an early ASP acquired by IBM in 2005. In fact, back in graduate school at Cornell, I did research on assembling commodity hardware into compute grids. Small wonder, then, that Cloud Computing is an area I'm looking at quite actively for potential investments.
Peter Laird has a great blog post that defines the landscape of Cloud Computing; I encourage anybody interested in the space to read Peter's post. And Peter's cheat-sheet on the players is invaluable if you want to appear well-informed :-)