TTL, Totem Pole vs. Open Collector Output
Totem Pole output drives the output high and low.  Open collector will only pull the output low; it does not drive the output high, only releases the output to float.

Multiple open collector outputs may be tied together to operate in a wire-OR arrangement, where any output may pull the output low.  In this case, a pullup resistor is used to establish a logic 1 when all outputs are off.

Open collector may also be used to drive non-gate loads, such as LEDs, small solenoids, etc., as long as the load voltage and current is within the rating of the open collector device.