Historians are software applications that log data coming from a control system.  Most DCS and PLC systems have their own proprietary historians but there are a number of 3rd party companies that produce historian software.  The software is usually classic-server client technology with a data link to the PCS.  Most data links are OPC connections that are connected to one of the PCS HMI's or Engineering Workstation.

This link should be fire walled and ideally only allow data to flow from the PCS to the Historian.  The Historians generally have access to the business layer of the plant and effectively allow data to flow up from the plant floor.

This data is very valuable to the business side of the plant and can be used as the base in many applications.

Table of Historian Suppliers