In electronic part seller websites, I see that there are two separate categories for opamps and comparators. As far as I know, an opamp itself is already a comparator if you don't connect a negative feed back and run it in the open loop mode. So, what are these "comparators" exactly? What makes them different than ordinary opamps? When should I prefer a comparator over an opamp?
Opamps are optimized for linear operation, in which the voltage difference between the input terminals is kept very small via feedback. As a consequence, the performance when using them in a nonlinear or open-loop application tends to be poor. In particular, charge storage on internal nodes tends to cause opamps to respond very slowly when coming out of an "input saturated" condition.
On the other hand, comparators are optimized for speed at the expense of linearity, and are designed specifically to be fast over a wide range of differential input voltages.