The Soul Shard is a required reagent for many warlock spells.
Acquiring shards
Soul Shards can be obtained by channeling the 
[Drain Soul] spell on a mob or a player from which the caster gains Experience or 
[Honor] by killing. If the mob dies while under the effect of Drain Soul, the player is granted a Soul Shard. The Warlock talent skill 
[Shadowburn] requires the use of a Soul Shard when cast; however, if the target dies within five seconds and yields experience or honor, the player gains a Soul Shard back. Since the rank of Drain Soul does not affect the ability to gain a shard, some warlock players conserve mana by using Drain Soul (Rank 1). These skills can be used in PvP.
Usage
The following spells consume a shard when successfully cast:
[Summon Voidwalker]- [Summon Succubus]
- [Summon Felhunter]
- [Summon Felguard]
- Enslave Demon
- [Create Healthstone]
- [Create Soulstone]
- [Create Firestone]
- [Create Spellstone]
- [Ritual of Summoning]
- [Shadowburn]
- [Soul Fire]
- [Ritual of Souls]
- [Soulshatter]
Heavy usage
The usage of shards varies between warlocks given their particular talents.
Destruction Warlocks use [Shadowburn], an instant-cast direct-damage spell that consumes a Soul Shard when cast. If the target dies within 5 seconds, and gives experience, the caster gains a replacement Soul Shard.
Demonology Warlocks tend to use slightly more Soul Shards for summoning demons than other warlocks, as they tend to summon more frequently, or use Enslave Demon more often. Also, with Improved Stone talents, Demonology Warlocks may be more apt to use Healthstones, Spellstones, Firestones.
Raiding Warlocks use Soul Shards to create Healthstones for their raid members, and to summon other raid members with 
[Ritual of Summoning].
Storage
Players can use soul bags to specifically hold Soul Shards in. Their benefit is that they are larger than comparative generic containers, and they automatically sort all incoming Soul Shards to that bag. Their icon on the container bar will have the number of shards remaining displayed upon it.
Notes
It is advised to keep a bag full of these at hand. While leveling, it may be a good idea to keep some in the bank because soloing with the Imp to get a shard can be quite annoying, and sometimes difficult.
A warlock can refund a soul shard of a summoned demon by setting the demon to stay and then gaining distance from it. When the distance is great enough, the demon despawns and a soul shard is returned. This only works for demons that cost a shard to summon.