Pulsed Lasers vs. Regular Lasers

A pulsed laser can produce extremely high peak power outputs. There are a number of pulsed lasers that can produce terawatt-levels of power for very short intervals -- a few nanoseconds at most, but their average power output is usually relatively low.

"Regular" (aka CW, for Continuous Wave) lasers can have high average power, but don't produce peak power levels even close to those produced by pulsed lasers. High power pulsed lasers are often "Q-switched." With these lasers the cavity's Q (the sharpness of the cavity's resonance curve, more or less) is kept low to allow the amplifying medium to fully "charge." Without a Q switch, the medium would begin to lase before it was fully charged and so would not be able to reach the high power outputs of its Q-switched cousins.

Among the world's most powerful pulsed lasers are the so-called Nova and Shiva lasers used for fusion research. These lasers are physically huge (occupying a space comparable to a large gymnasium) and can theoretically reach peak power levels in the petawatt range.

Why do laser sensors used for distance measurement applications use pulsed laser and not continuous laser ?

These lasers send a pulse and then wait for its return. The moment the laser is pulsed a timer is started, and when the reflected pulse returns the timer is stopped. The distance to the target is then calculated as the speed of light times the interval divided by 2. Divided by two because the round-trip distance is twice as far as the distance to the target.

Radar and sonar work in a similar way and a "radar" that uses light instead of radio waves is called a lidar.