Also referred to as a “stopwatch” function, and not to be confused with a chronometer, a chronograph typically uses subdials to keep track of seconds, minutes, and hours. A chronograph minutes/hours display can be designed to run on the central hand stack instead of subdials, however, it is most common for only the chronograph seconds hand to be centrally mounted.
There are two common types of chronograph “cam and lever” actuated and “column-wheel” actuated. Cam actuated chronographs tend to be seen on more accessibly priced watches and column-wheels on more pricey timepieces. Column-wheels, if designed well often deliver a more smooth push actuation for start, stop, and reset. While cam operated pushers tend to have a bit harder push actuation.
With both cam and column-wheel chronographs there are also two other important variations “horizontal” or “vertical” clutches. Horizontal clutches are designed in a way that causes the chronograph seconds hand to jump slightly at the start whereas vertical clutches that mesh with the gears continuously do not cause a jump. This design also makes it so the running of the chronograph does not adversely affect the timekeeping of the watch, whereas with a horizontal clutch the timekeeping is adversely affected somewhat when running the chronograph.
As far as reliability, cam or column-wheel, with either horizontal or vertical clutches, are all generally considered to be equally reliable.