working of opto isolator

Hi,

I studied that a combination of an LED and a photo detector in two separate circuits could be used as an opto isolator where the light emitted by an LED could be used as excitation source to the circuit containing photodetector.

My question is that can the circuit containing the photodetector could be any electrical circuit or it necessarily has to be a circuit containing only one resistor and a voltage source?

Basically I want to know the applicability of the opto-isolator circuit. Does this circuit represent a family of circuits or it can be any particular circuit that we need to mug-up?

regards,
Proffy