In short
OT refers to field systems that monitor and control physical processes (PLC, SCADA, DCS); IT refers to enterprise information systems that run business processes.
OT prioritises uninterrupted, safe operation; IT prioritises data integrity and confidentiality. Industry 4.0 projects require these two worlds to converge securely (OT/IT convergence).
Good practice keeps OT and IT networks in separate segments (VLANs) with access only through a controlled gateway.