An open collector output can onliy drive the output pin to GND. That is: when the output is logic zero the pin is connected to GND and when logic one the output pin is floating (high impedance state).
Normal TTL outputs can drive the output pin to GND (logic zero) or to VCC (logic one)
OC outputs have only one transistor (it may be a FET in CMOS logic) . The collector is connected to the output pin.
Normal TTL outputs have 2 transistors (one to VCC and the other to GND) which is known as a push-pull or totem pole output.