Zookeeper
In plain terms
A coordination service for distributed systems.
Going deeper
Apache ZooKeeper manages configuration, naming, synchronization, and leader election for distributed applications. It provides reliable coordination primitives.
Under the hood
Offers consistent, ordered state via a quorum; historically backed Kafka and Hadoop, now sometimes replaced.
Examples
Distributed coordination, leader election, config management, Kafka (legacy), Hadoop.
Think of it like...
Like a referee keeping all the distributed players in sync and agreed on the rules.