SharePoint Central Administration (the CA) is a web application that typically exists on a single server in the farm.

SharePoint Central Administration (the CA) is a web application that typically exists on a single server in the farm, however it is also able to be deployed for redundancy to multiple servers.

Questions having the Central Administration tag, refers to configuring, implementation and use of Central Administration.

Reference: http://en.wikipedia.org/wiki/SharePoint#Administration_and_security