VMRole is another PaaS offering from Microsoft on the Azure platform. Like the Web/Worker role it is a Stateless machine. However developers are given more control over customizing the OS by supplying a preconfigured VHD image that is used to run the VMRole from.
Windows Azure VM Role is a Platform-as-a-Service (PaaS) offering from Microsoft. Like the WebRole and WorkerRole it is a stateless role, thus PaaS. The advantage of this type of offering is that one can avoid long-running installs of third party software, which usually runs in the Startup for a Web/Worker Role. Also the VM Role gives more freedom on customizing the OS that runs in the cloud (though currently only supported Windows Server 2008 R2 and Windows Server 2008 SP 1 x64). The role is being spinned up from preconfigured VHD image. One shall not forget that the VM Role is a stateless. This means that any and all changes made during the live of a VM will be lost upon service healing (if hardware fails and needs to be re-instantiated on another machine) just like the Web/Worker roles. This is true for the file storage also.
It shall not be confused with Windows Azure Virtual Machines, that came as offering in June 2012 and is still in CTP as of today (Sept. 2012).
More information on VMRole and Virtual Machines can be found here.
