Chand Baori is a famous stepwell located in the village of Abhaneri near Jaipur in the Indian state of Rajasthan. It is situated opposite of the Harshat Mata Temple and was constructed in the 9th century. Chand Baori consits of 3,500 narrow steps over 13 storeys. It extends approximately 100 ft into the ground making it one of the deepest and largest stepwells in India. [Source]

Stepwells, also known as kalyani, pushkarani, bawdi, baoli, barav, or vaav; are wells or ponds in which the water may be reached by descending a set of steps. They may be covered and protected and are often of architectural significance. Most common in Western India, they can also be found in arid regions of Pakistan.

The basic difference between stepwells and tanks or wells is that stepwells are easier for people to reach the ground water. They are also easier to maintain and manage. Stepwells also served other purposes, such a relief from the daytime heat and as a place for social gatherings and religious ceremonies. [Source]