The Petrov Defense is a chess opening played by black in response to 1. e4 e5 and 2. Nf3. Black responds with Nf6 instead of Nc3. Black counter-attacks the pawn in the center and poses threats of his own. This opening is a little dangerous for black, especially if correct play doesn't follow. Though this symmetrical response has a long history, it was first popularised by Alexander Petrov, a Russian chess player of the mid-19th century. In recognition of the early investigations by the Russian masters Petrov and Carl Jaenisch, this opening is called the Russian Game in some countries.
Here is a list of the 4 most popular continuations in the Petrov Defense:
a) Classical Variation
b) Steinitz Attack
c) Urusov Gambit
d) Damiano Variation