In the output circuit that uses a transistor, the collector is an output terminal. The output usually consists of an external pull-up resistor, which raises the output voltage when the transistor is turned OFF. When the transistor connected to this resistor is turned ON, the output is forced to nearly 0 volts.
In the open collector output circuit, the power supply is required, but the voltage level can be set freely in the range.
Open collector output circuit
Recommended circuit (reference)