op amps comparator or differential amplifier
When you see an opamp circuit and ask yourself is this open loop / a comparator or not?
Then first see if there is feedback.
If there is no connection back to the input(s) then it is a comparator:

Source
If there is feedback it can still be a comparator (possibly with hysteresis) but it can also be a linear amplifier.
Now check if the feedback is positive (feed forward) or negative (feed back).
Here's an example of a comparator with positive feedback meaning it has hysteresis:

Note how the signal going back goes to the + (positive) input of the opamp.
This is an example of negative feedback so this is a linear amplifier:

Note how the signal going back goes to the - (negative) input of the opamp.
In your circuit there is a PNP transistor in series with the feedback. (Next time, draw transistors with the arrow pointing down as that indicates the direction of the current flow which is from the positive supply down to ground.) This PNP is in a common collector configuration also called emitter follower. A property of that configuration is that it does not invert the signal (which common configuration does invert the signal?) so that means that the feedback isn't inverted and since the signal feeds back to the - input of the opamp, this is a linear amplifier circuit.
That means that the opamp will try (and when the circuit is dimensioned well, the opamp will succeed) and make the voltages at its + and - inputs the same. That means that the voltage across zener diode D1 is copied across Resistor R2.
I would draw the circuit like this as that makes it easier to understand:



simulate this circuit &ndash; Schematic created using CircuitLab
The result of that constant voltage across R2 is that the current through R2 will be constant as well. That constant current flows through Q1 into R3 which is the load. You can change the value of R3 (within practical limits) but the current through R3 would remain constant. This circuit is called a current source or current reference.