Grid computing is one of the most complex ways of hosting. You have to think of Grid computing as a distribution system that has no interactive parks with a large number of files involved. Grid computing is different from other high performance computing systems like cluster hosting because grid computing tends to be more loosely couples, diverse, and widely dispersed. Sometimes one grid can be used for a single application but most of the time they are used on multiple applications.
Grids can vary in size by an extreme amount. Grid computing is really similar to a super computer but most of the time is not as constrained as a super computer. They can perform very large tasks just like a super computer can. Most grids can be seen as a type of parallel computing because each computer is a complete computer with the normal CPU, hard drive, power supply, network interface, RAM and others. What makes grid computing different is that each computer communicated over some type of network such as Ethernet and can be communication over long distances. This is different from a super computer because a super computer is normally couples closely and in on one high speed local computer system.
As far as hosting companies that use grid hosting/computing there are many. A couple of the more notable companies are listed below.