The Reti Opening (1. Nf3 d5 2. c4) belongs to the Hypermodern School of Chess that dominated in the early 20th Century. The idea is to control the center rather than contest it by direct occupation with pawn moves such as d4 and e4. Because of its fluid nature, Reti Openings often transpose into variations of the English, Queen's Gambit or other closed games.
The most popular 2nd moves by Black are e6; c6; d4; and dxc4.