There are two main types of input/output modules for a PLC: analog and digital. Analog modules handle continuous signals like 0-10V, 4-20mA, and -20-20mA, while digital modules handle on/off signals represented as 1s and 0s. Common digital inputs include push buttons, switches, and sensors, while common digital outputs include relays, contactors, and lamps. Analog inputs include temperature and pressure sensors, and analog outputs control actuators, valves, and drives. The next video will discuss PLC programming languages.