ENCYCLOPEDIA OF CHESS OPENINGS
ECO stands for Encyclopedia of Chess Openings and is the standard way of listing all the openings. 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