You're doing well Doddy! I spent 16 years working out how to produce clear, easily understandable explanations of electronics stuff and it's not as easy as you think it is.

Nemo1966,
Never be embararrased to ask the simplest questions. I guarantee that there will always be a dozen or more other people thinking "I wanted to know that but was afraid to ask".

The only thing I would add about open collectors is that they are designed to be flexible. Unlike an output which switches from 0-5V, 0-12V etc. which restricts the voltage used by the external device, an open collector (especially the more common NPN version) can be used with any external device within the voltage and current capability of the transistor regardless of the supply voltage used by the device containing it.

Kit