ECO stands for Encyclopedia of Chess Openings and is the standard way of listing all the openings. History wise so far all I've been able to find is that it was first compiled sometime in the seventies, was edited by a player named Matanovic and compiled by some of the better players of the time (perhaps called the Chess Informator Team). It is the standard. I also believe they release new more Extensive ECOs but the main stuff listed below never changes.

<>A is stuff like English Opening, Reti, Benoni,Dutch Defence. . .

               A  A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 

<>B is stuff like Sicilian Defense, Caro-Kann, Pirc, Modern, Alekhine, Scandinavian
               
               B  B0 B1 B2 B3 B4 B5 B6 B7 B8 B9

<>C is stuff like Ruy Lopez, The French, King's Gambit

               C  C0-C1 C2 C3 C4 C5 C6 C7 C8 C9 

<>D is stuff like Queen's Gambit, The Slav, Gruenfeld, Queen's Pawn Game...  

               D  D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 

<>E is stuff like King's Indian, Nimzo-Indian, Queen's Indian

               E  E0 E1 E2 E3 E4 E5 E6 E7 E8 E9