Hindustani is a name for what is arguably a single language, but is generally treated as two distinct languages:
While the spoken forms of the two are close enough to be mutually intelligible, there are differences; most notably, they are written entirely differently. Hindi uses an alphabet derived from Sanskrit (the language of the ancient Hindu texts) and is written left-to-right. Urdu uses a Persian variant of the Arabic alphabet and is written right-to-left. Urdu also uses more loanwords from Arabic, Persian and Turkish, though Hindi has some as well.
The term "Hindustani" was fairly common in the days of the British Raj, but has mostly fallen out of use today. One place where it is still sometimes used is to describe the language used in "Bollywood" films, produced in India — mostly Mumbai; Bollywood is from Bombay plus Hollywood — but also popular in Pakistan.