This is going to be an article on the Indian defense. It is the most common reply against 1.d4 by white. It is a rather modern defense, and up until 20th century it was regarded as irregular. This opening is also transpositional, for example black can play e6 and d5 while white does c4, leading to a Queen's Gambit declined.
Nf6 is a flexible move. It keeps open options for transpositions, and leads to a complex of openings called the Indian defenses, most of which have been covered in other articles. They include the Bogo Indian, Queen's Indian, King's Indian, and the Nimzo Indian. More openings arising from this include the Benko and Budapest gambits, Modern and Czech Benoni, Gruenfeld, and some more less played ones! Thanks for reading and enjoy!