Platform as a Service

What is a platform as a service? And what problem is it trying to solve? I think thagh, the term itself has not existed for as long as som people realize. The way i think of soft ware as a service is you're actually just running softwere on line without having to down load something to your local system. So that's that softwere is a service. Then at the other end, you have intra structure as a service. That's most of the ploud computing provider.