MSDP stands for MUD Server Data Protocol. It allows the transparent sending and receiving of variables between the server and client, providing a useful tool for scripts and plugins.
MSDP is supported by MUSHCLIENT, allowing you to benefit from its features.
Read more about it here: http://tintin.sourceforge.net/msdp/
MSDP is also used by the official God Wars II plugin (see ‘help gui’).