Baolis or step-wells were once common features across the more arid parts of India, including Delhi, Haryana, Rajasthan and Gujarat. The basic feature of a baoli is that it consists of a series of steps leading down to the water level: as the water level recedes over time due to use or evaporation, you must climb down more steps to get to the water. Delhi has several step-wells, such as the ones at Nizamuddin and at Purana Qila; Agrasen ki Baoli, Gandhak ki Baoli (also in Mehrauli, and approachable through the Mehrauli Archaeological Park), and this one, named for stonemasons or raj.
Besides the main body of the baoli (which is surrounded on either side by shallow cells or rooms that could be used for rest), there is a pillared pavilion up above.
When we visited, the main gate was locked and we were not allowed to go in. The water in the baoli was almost up to the level of the topmost step, and looked very green and stagnant.