Publicly available cloud computing began with the launch of Amazon's Elastic Compute Cloud, offering Infrastructure as a Service. Soon afterwards, Google launched its Google App Engine, which took a different path, offering a development Platform as a Service. In due course, Microsoft took a path similar to Google's, with its recently launched Azure platform. We first present a user's picture of each of these major public cloud offerings. Next we consider the economics of cloud computing, again from the perspective of an enterprise user. We examine whether, when and why using a public cloud for enterprise applications might be cheaper than deploying the same systems in-house, i.e., in a ‘private’ cloud.