Well instead of going in to any applications, in theory positive feed back is when the output is fed back to the input and it adds to the input or called in phase with input. So generally this leads to increasing input to the system as time progresses. With negetive feed back you have the same feed back as before but intead the feedback is out of phase with the input, so it subtracts from the input or reduces the input to the system. Now this feed back can be either voltage(more common) or current or any quantity as such where input is converted to output.
Opamp is one of typical examples of the feedback system. But it can be any sytstem other than electrical also, even biological.