A post office or postal system is a good way to picture it. A nice safe secure system for communicating between various people (systems/ applications/ etc).
It can be used anywhere you don't need instantaneous one to one requests/replys, when batched processing can be employed - though it can handle pretty quick replys.
I can protect the data across the LUW using commit/rollback.
And one of its greatest strengths is that it is almost platform/operating system independent.