The french defense is one solid opening black can play against white's e4. This opening (1. e4 e6; 2. d4 d5) is criticized for momentarily blocking black’s queen's bishop causing delay in the development of the pieces but at the same time creating solidity that is very difficult to destroy with normal play.
With the momentary blockade of the queen's bishop, black's position looks cramped and may result to passivity. If black fails to solve this problem, he could remain on the defense during the entire game. The attacking possibilities for black usually initiates on the queenside where he has a seeming majority given the resulting pawn structure in the event that white pushes his pawn to e5 -characteristics of the “advance variation” and the “Winawer”.
In the advance variation of the French defense where white is able to occupy the e5 square with a pawn, at first look, appears to be an advantage as the pawn push gains space for white and denies black's knight the coveted f6 square. But in reality it is very difficult for white to hold on to his e5 pawn. Black's move (c5) is a thunderous declaration of the intent to challenge white's pawn chain. Once this pawn chain is destroyed, the e5 pawn is dangerously holding for its dear life.
To avoid this scenario of losing the e5 pawn after a voracious attack from white, the exchange variation is a great alternative. In this variation, white immediately exchanges his e4 pawn with black’s d5 - the pawn structure is eradicated, tension is released in the center but an early scuffle usually results in both ends of the board.
White's attacking prospects are on the kingside and it is usual to see an attack by black on the other side of the board while white is pursuing his. In several games, I have seen an early Qg4 with focus on black’s g7 pawn making it very dangerous for black to castle kingside.
Aside from being known as a solid opening, the french defense is reputedly resilient - a characteristic viewed by some chess players as "dullness". After the moves (1. e4 e6; 2. d4 d5) the pawn structure created is at first impression "dull" compared to the highly energetic nature of the ruy lopez.
If we look closer however, the french defense is a very dynamic opening where sudden changes in the board situation can occur dramatically with just mere exchanges of pawns. In the Winawer, white cordially accepts a damaged pawn structure on the queenside with the hope of holding the seeming majority of space in the center. Due to the damaged pawn structure, white more often than not, castles on the kingside to avoid black's onslaught on the queenside.
During those times when the kingside is already compromised and white has gained a very promising attack on this side of the board, black always has the option of castling queenside which results to a very exciting chess game.
Aside from the sicilian, the french defense is one opening that is difficult for white to face. The move (e6) as a reply to white's e4 is a curtain opener to other various solid openings at black's disposal including the dreaded pirc defense.
With the french defense, black can be said to be shooting two birds with one stone since he gains solidity against premature attacks and the same time flexible enough to meet the changes in the situation during the early stages of the game.