ECO Codes

ECO (Encyclopedia of Chess Openings) has created a method of categorizing all the openings into alpha-numeric codes. Variations of an opening will have numeric extensions on the 3rd digit.

Using the ECO Code, it is possible to search through Databases for a particular opening or variation. Broadly, there are 5 alphabets into which all openings are classified:
A - Flank Openings (English, Benoni, Benko, Dutch, Reti)
B - Semi-Open (Sicilian, Caro-Kann, Pirc, Alekhine, Center-counter)
C - Open (Ruy Lopez, French, Petroff, Philidor, Scotch, Kings Gambit, 4 Knights, Giuoco Piano)
D - Semi-Closed and Closed (Queens Gambit, Slav, Grunfeld)
E - Indian defenses (Nimzo indian, Queens Indian, Kings Indian, Catalan)

More Details on Wiki: Overview & Full-List

Detailed List with moves on WCCF site

Openings list with Variations on LaMecca

Chess Informant Home Page