The official MCP website can be found at www.moo.mud.org/mcp/.
MCP is a protocol that is aimed at being simple to use and that integrates well with an existing server. It specifies an out-of-band system of messaging where the protocol messages can be present in the same transport channel as the regular mud data. By doing so, it doesn't require replacing or rebuilding everything to do everything over a custom protocol.
MCP specifies a mechanism for exchanging messages between the client and server, but not the specifics of those messages, which are left to the application designer. To support this, MCP specifies a system for naming packages as well as negotiation between the client and server to determine which packages each supports (including compatible versions of those packages).