ECO in Chess

ECO ( or Encyclopaedia of Chess Openings) is a classification of chess openings moves.The moves were taken from hundreds of thousands of games between masters, from published analysis in the Chess informant since 1966.
Instead of the traditional names for the openings, ECO has developed a unique coding system that has also been adopted by other chess publications. There are five main categories, "A" to "E", each of which is divided into one hundred sub-categories.
ECO code is a trademark of Chess Informant Inc.

Category A contains the openings which are considered as flank openings( i.e the attack using the minor pieces rather than the pawns such as in the e4 or d4 openings).
they also include the unusual openings which are denoted by A00.

Major openings that are used are:

1. English Opening ( A10 to A39 )

2. Nimzovich-Larsen Attack ( A01 )
3. Benko Gambit

 Category B

These include the semi open games excluding the french opening. (A Semi-Open Game is a chess opening in which White plays 1.e4 and Black breaks symmetry immediately by replying with a move other than 1...e5. The Semi-Open Games are also called Single King Pawn, Asymmetrical King Pawn, or Half-Open Games)

The openings that are included are 

1.Sicilian Defence ( B20 through B99). This opening is given the highest number of codes.
2.Caro-kann Defence( B10 to B19)
3.Alekhine's Defence (B02 to B05)
4. Modern Defence 

Category C   

This category includes the open games and the french opening. ( Open games are the games starting with 1.e4 e5) ( open games can also refer those games in which the gameplay tactics favour along the diagonals)

The opening in ECO C include
1. Ruy Lopez
2.French Defence
3. Italian Games
4. Kings Gambit  

Category D

This category of ECO codes include the Closed game and semi closed games.

Closed game
: is a chess opening that begins with the moves:

1. d4 d5

Semiclosed Game: in which White plays 1.d4 but Black does not make the symmetrical reply 1...d5.

The openings that are included are:

1. Queens Gambit: Accepeted/ Declined
2.Grunfeld Defence

Category E

ECO includes the Indian defences.

Indian defences are seen by the moves 
1.d4 Nf6

The opening include
1. Nizmo Indian defence
2.Kings Indian defence
3.Queens Indian Defence
4. Catalan Opening.

Links and credits:
Chess informant