Amaranth, or Rajgira, is a pseudo-grain, closely related to quinoa. It grows all throughout the world. Even though it seems to originate in the New World, it has been in India since pre-Columbian times.
In Indian food we mainly use amaranth as a grain. I like to make a sweet with it, Rajgirani chikki, especially to celebrate Kite Festival. It can also be made into a porridge, or ground into flour that is used to make rotis.
All parts of amaranth are edible. The leaves can easily be used in any recipe that calls for greens, such as saag paneer.