Metal as a Service is a new layer in the stack under IaaS and is described by Mark Shuttleworth as, “…bringing cloud semantics to the bare metal world.”
Essentially MaaS supplies bare metal that can be deployed similarly to cloud instances, however you are actually deploying physical nodes. Traditionally deploying hardware is time consuming and technically complicated, MaaS simplifies this and makes it possible to interact with physical devices similarly to AWS instances (with a greater time frame and without automated API deployments).
Canonical invented and were the first to market with an offering like this, and have integrated the JuJu deployment system with MaaS. Using JuJu, operating systems and services can be deployed allowing rapid scaling without the need for a senior engineer or network architect. JuJu is similar to Chef or PXE and in a similar way allows the rapid and simple interconnection of services across multiple physical hosts.