A PLC is used to control a PROCESS.
It can be used to control very simple processes or very complex process depending on the application.
Examples:
Paper Mill, Saw Mill, Sewage Treatment Plant, Robotics etc, Wharf Cranes, little machines, big machines.... the options are enormous.
If we are talking about a plant, a plant will usually have more than one PLC with each PLC controlling a different section of the plant.
These PLCs are usually networked together.
A PLC is used to replace physical wiring and relays in a control switchboard meaning that instead of hard wiring all the sensors, switches etc into a control board and then rewiring the whole board every time we want to make a change, all we have to do is re programme the PLC.
This saves time and money...and mess if you look at any boards I rewired that dont have a PLC.
Basic Concept:
You have something needing to be controlled. Your front porch light.
It needs to come on at 8pm and off at 10pm or if an intruder passes a sensor, but only if the temperature is above 60degs and you want an override switch so you can turn it on any time you want.
The intruder sensor, the temperature sensor and the override switch are inputs INTO the PLC.
The porch light is an output FROM the PLC.
The timer is integrated in the logic that you program into the PLC
using a computer with the appropriate software, and a cable to connect between the PC and the PLC.
As far as a car goes...
Think - temperature, oil pressure, rpm, speed, torque etc are inputs to the PLC
Think - Fuel injectors, warning lamps, buzzers, solenoids, etc are outputs from the PLC.
The manufacturer programs into the PLC the logic that exists between the PLC inputs and outputs. eg. "If my speed is 50mph, and my engine torque is X then I am running inefficiently so adjust my fuel to compensate.
Dont know if I'm on the right track for what you want. If you elaborate a bit more, others can help.