DCOM is the acronym for the Distributed Component Object Model, an extension of the Component Object Model (COM). DCOM was introduced in 1996 and is designed for use across multiple network transports, including Internet protocols such as HTTP. DCOM is based on the Open Software Foundation's DCE-RPC spec and will work with both Java applets and ActiveX components through its use of the Component Object Model (COM). It works primarily with Microsoft Windows.
More information: DCOM page on the Microsoft website
More on the general topic: Prior Service-Oriented Architecture Specifications