The Warlock requires Soul Shards for certain spells and pets. You receive a Soul Shard by using the Drain Soul spell on a monster as it is dying. A catch is that you cannot get Soul Shards from monsters that don't give you experience. So you can't go into a newbie region and kill easy monsters for Soul Shards. One Soul Shard takes up one slot in your bags, which means you don't want to carry a whole lot of them. However, you do always want to have some in your bags so you can summon your pets. When using Drain Soul the caster cannot do any other action such as move, attack, or cast another spell, or else the spell will be interrupted. If you do not stand still, the Drain Soul will not work. You will need to learn how to do this properly, which can be challenging.

A Soul Shard will be refunded to the caster any time a summoned pet despawns rather than dies.

Soul Shards do not stack. This is for balance reasons and so you won't hoard huge numbers of them.

If you find yourself without Soul Shards, you will have to get them without the use of most pets, which can be a challenge. Imps can be summoned without the use of a Soul Shard so you can use them to get your Soul Shard, and then switch to a more powerful pet.
Honorable kills generate a Soul Shard when Drain Soul and Shadowburn are used.
Warlocks will be credited a Soul Shard from their pet when they take a flight path.
Warlocks will be credited a Soul Shard from their pet when they are dead at the end of a Battleground match. Warlocks will be credited a Soul Shard from their pet when they enter an Arena.

Soul Shard Bag

Tailors can make soul bags to hold soul shards. The smallest is a 20-slot bag, the recipe for which can be purchased in Gadgetzan. The recipe for a larger bag can be found in Scholomance, and one for an even larger bag can be found in Molten Core. In the main UI, these soul bags will display how many shards they contain. So long as the soul bags have room, any soul shards a warlock creates will automatically go into the bag.