As part of the Microsoft Open Protocols program, SharePoint Products and Technologies Protocols documentation has been published (updated) recently. This documentation provides detailed technical specifications for Microsoft proprietary protocols, including extensions to industry-standard or other published protocols, that are implemented and used in SharePoint Products and Technologies to interoperate or communicate with Microsoft products. The documentation has 2 broad parts:
SharePoint Front-End Protocols - Specifies the protocols implemented by Microsoft SharePoint Products and Technologies to communicate with client and server applications. These protocols are intended for interoperability between SharePoint Products and Technologies and external applications
SharePoint Back-End Protocols - Specifies the protocols used by SharePoint Products and Technologies for internal communication. These protocols are intended for interoperability between components of SharePoint Products and Technologies, or third party components implementing some or all of the functionality of a SharePoint component
Download URL: Microsoft SharePoint Products and Technologies Protocol Documentation
If you are interested in knowing the nuts and bolts of SharePoint, there is tons of interesting stuff in the documentation. An example is - [MS-FSSHTTP]: File Synchronization via SOAP over HTTP Protocol Specification. Word 2010 Technical Preview enables multiple users to simultaneously edit and save changes to a document that is stored on a WSS 4.0 Technical Preview server. This feature is called co-authoring. The [MS-FSSHTTP] protocol is used to enable co-authoring.
[MS-FSSHTTP] protocol enables a protocol client (e.g. Word 2010) to call a cell storage service request that allows for upload or download of file changes, along with related metadata changes to or from a single protocol server (e.g. WSS 4.0). In addition, the protocol server processes different types of locking operation requests sent by a client, that allow for uploads to be done while preventing merge conflicts on the shared resource. The protocol is a request/response stateless message exchange protocol based on SOAP that uses HTTP 1.1 for its transport and SOAP Message Transmission Optimization Mechanism (MTOM) encoding. Read the 106 page PDF document for complete details :-)