What is soul shard?

In the game World of Warcraft a warlock will use a soul shard to perform certain tasks ie: the creation of soulstones, healthstones or to summon a demon (except the imp and infernal). They are also used to summon a party member to the lock's location.
In order for a lock to get a ss they must use drain soul while yielding xp during the death of a player or npc.