I'm going to assume you mean Soul Shards, but I'll explain them both anyways
1. SoulStones
At a certain level you get the ability to make a Soulstone for one soul shard. You can use the stone on yourself or a party/raid member. The stone will allow the person to resurrect right after death for 30 minutes. The ability itself has a 30 minute cooldown.
2. Soul Shards
Soul Shards are required to summon all demons besides the imp, make all types of stones, and summon people with ritual of summoning. It is also used for Soulfire and Shadowburn.
You get soul shards by using drain soul. If you are using drain soul while a mob or player that gives you exp/honor (Green or above) you will get a soulshard. Shadowburn returns a soulshard if the target dies within a certain amount of time after it's cast.