Your web app needs to know what IP address, username and password to connect to the database on.

If it's all one the same box, that's trivial.

But imagine you have several app serves, and several other systems that all needs to talk to the database. Then one day you need to change details of the database server. Now you can either use a tool to update all the configurations (or do it by hand), or you can have all of these apps pull the database config from a distributed directory of some sort.

Etcd is such a directory. Similar to LDAP, but simpler (LDAP requires you to write schemas, and decide which attributes to index on and in set up replication hierarchies). Some people also use DNS for this (you can put arbitrary data in a private DNS server).

Imagine you're working with a processing system made up of tens or hundreds of computers all (usually) identically configured, networked together and solving small chunks of a large problem. This is software to minimize the effort required to configure all of those individual systems which make up the "compute cluster".

Zookeeper is a very specialized distributed filesystem for saving and delivering important details for very large systems. Getting concurrent distributed systems like this right is the equivalent of summited Mount Everest starting by walking from Paris. It was mostly developed at Yahoo and spun off as an Apache project under Hadoop and commercialized by Cloudera. Netflix uses ZK for example.