A shard is a physical division within a keyspace; i.e. how data is split across multiple MySQL instances. A shard typically consists of one MySQL master and one or more MySQL replicas.
Each MySQL instance within a shard has the same data, if the effects of MySQL replication lag is ignored. The replicas can serve read-only traffic, execute long-running queries from data analysis tools, or perform administrative tasks.
An unsharded keyspace always has only a single shard.