Wikipedia
OpenDoc was a multi-platform software componentry framework standard for compound documents, intended as an alternative to Microsoft's Object Linking and Embedding (OLE).
The basic idea of OpenDoc was to create small, reusable components, responsible for a specific task, such as text editing, bitmap editing or browsing an FTP server. OpenDoc provided a framework in which these components could run together, and a document format for storing the data created by each component. These documents could then be opened on other machines, where the OpenDoc frameworks would substitute suitable components for each part, even if they were from different vendors. In this way users could "build up" their documents from parts. Since there was no main application and the only visible interface was the document itself, the system was known as document centered.
It was envisioned that OpenDoc would allow smaller, third-party developers to enter the office software market, able to build one good editor instead of having to provide a complete suite.