my two cents worth ~
Cloud refers to an (imaginary/easily scalable) unlimited space and processing power. The term shields the underlying technologies and highlights solely its unlimited storage-space and power.
Grid is a group of physically close-by machines setup. Term usually imply the processing power (ie:MFLOPS/GFLOPS), referred by engineers
Cluster is a set of logically connected machines/device (like a clusters of harddisk, cluster of database). Term highlights how devices are able to connect together and operate as a unit, referred by engineers