I/O (or Input/Output) commonly refers to the data points of a PLC. Specifically, the input and output wires of a PLC are directly connected to a nearby machine. Usually Ignition will interact with other memory locations in the PLC, but sometimes users want to see the exact value coming from a machine.
Industrial control system is a general term that refers to several types of control systems used in industrial production, including supervisory control and data acquisition (SCADA) systems and smaller control system configurations such as programmable logic controllers (PLC) often found in the industrial and critical infrastructures.
An Integrator is a person or company who specializes in integrating control systems into manufacturing processes. Integrators are the heart of Inductive Automation, they are the ones called on to create an Ignition project in a facility or expand existing systems.