During the good old days of WinNT, MTS used to provide the middle layer for distributed computing using MS technologies. The MTS (Microsoft Transaction Service) environment used to sit in between the client and database and provide distributed transaction services, object pooling, security and other services.
With Windows 2000, MTS was upgraded to COM+. While MTS was actually an add-on to Windows NT, COM+ was natively supported by Windows 2000. This simplified the programming model quite a bit. It was often said that MTS + COM = COM+.
Thus MTS/COM+ is the equivalent of EJB in the J2EE world.
In .NET, COM+ components can be written as .NET component services. The "System.EnterpriseServices" namespace provides us with classes that enable us to deploy a .NET component as a COM+ component. Existing COM+ components can be accessed using .NET remoting and COM-Interop.