Defining/articulating/explaining what cloud computing is seems to have become a cottage industry these days. Perhaps obfuscation allows many enterprises whose products/services are only peripherally aligned to the concept to climb on board what they perceive as the emerging engine in the technology marketplace.
From my perspective, the definition has always been pretty clear. Cloud computing really comes into focus only when you think about what IT always needs: a way to increase capacity or add capabilities on the fly without investing in new infrastructure, training new personnel, or licensing new software. Cloud computing encompasses any subscription-based or pay-per-use service that, in real time over the Internet, extends IT's existing capabilities.
James Staten, of Forrester Research, delivered a nice overview of cloud computing in a teleconference earlier this year. The slides from the teleconference still provide a nice primer for the technology.