What is PLC ? How it is useful in automations ?

PLC is a programmable Logic controller which is used mainly for interlocking different equipments.PLC using for particular machine or production unit.PLC is economically low cost.PLC'S can handle analog and digital I/O as earlier it could handle only digital.PLC'S are automatic controllers which is a substitute to hard wired controllers.they are extensively used for automation.PLC is for stand alone system.PLC is used Commonly with On\\Off (Digital) Control and may be expanded with Analog I\\O Modules for Analog Control and used for a control task.