What are the differences between a comparator and an op-amp?

While both are differential input devices, comparators are optimized for open-loop operation with fast response time (ns range), rail-to-rail output swing for digital compatibility, and no frequency compensation (would slow response). Op-amps are designed for closed-loop operation with internal compensation for stability, linear output range, and lower speed. Using an op-amp as comparator results in slow response and possible oscillation; using a comparator in linear mode causes instability.