Cloud is a marketing term, with the bare minimum feature relating to fast automated provisioning of new servers. HA, utility billing, etc are all features people can lump on top to define it to their own liking.
Grid [Computing] is an extension of clusters where multiple loosely coupled systems are used to solve a single problem. They tend to be multi-tenant, sharing some likeness to Clouds, but tend to rely heavily upon custom frameworks that manage the interop between grid nodes.
Cluster hosting is a specialization of clusters where a load balancer is used to direct incoming traffic to one of many worker nodes. It predates grid computing and doesn't rely on a homogenous abstraction of the underlying nodes as much as Grid computing. A web farm tends to have very specialized machines dedicated to each component type and is far more optimized for that specific task.
For pure hosting, Grid computing is the wrong tool. If you have no idea what your traffic shape is, then a Cloud would be useful. For predictable usage that changes at a reasonable pace, then a traditional cluster is fine and the most efficient.