By combining distributed computing resources into a single system "image," grid computing creates a virtual computer from which users can access data and resources as required.

For example, on distributed platforms, map-reduce coupled with in-memory databases; on System Z, co-deploy for access to enterprise data.

It's a distributed memory caching system which Facebook (and a ton of other sites) use as a caching layer between the web servers and MySQL servers (since database access is relatively slow).