Shard is the computer gaming industry term for one out of many servers organized in a way to handle distributed or decentralized gameplay in order to handle scalability issues, typically of MMORPGs.

I always thought these shards would solve the problem of having all user entities intercommunicate whenever necessary, not just for messaging but also for interaction in the game.

Apparently this isn't always the case. elrous0 says:

And, as a recovering KobraMUD addict myself, I can definitely attest that the classic MUD's were every bit as addictive as their modern MMO [short for MMORPG] counterparts. Though they required a little more imagination, their miniscule system requirements and free online play helped make up for it (not to mention that their low overhead allowed them to be played without shards or multiple servers--letting you play with all your friends without having to jump servers).

It's the first time I hear some games have you jump servers to be with your friends, thus not fully interserver capable. That's something PSYC would not want to happen to you.

See also: Space and Second Life.