Cloud computing vs. Grid computing explained in IBM's paper from 2009. It explains how cloud computing is an extension to grid computing. An evolution, a natural progression, if you want. The term 'utility computing' is also explained and this is what today's services are becoming more and more like - paying for what one uses on a shared infrastructure.
Cloud computing represents Infrastructure as a Service (IaaS), as opposed to Platform as a Service (PaaS) or Software as a Service (SaaS) concepts.
Further to that, some great Cloud computing resources available at