What is Sharding / Horizontal sharding?

A technique to partition a set of related data into smaller groups. Each group is called a shard, which has logically same schema as the unsharded data, the only difference is the data size. This technique is often to address scalability issue when a single database instance is incapable of serving all data, thus sharding is introduced to split the data and spread load across multiple instances. Sharding brings quite a lot overhead, so normally it's the last resort until a single instance reaches its scaling limit.