Smart Game Format
The SGF file format is designed to store game records of board games for two players. It's a text-only, tree-based format.
Games stored in SGF format can easily be emailed, posted or processed with text-based tools.
The main purposes of SGF are to store records of played games and to provide features for storing annotated and analyzed games (e.g. board markup, variations).
SGF files are comprised of pairs of properties and property values, each of which describes a feature of the game.
You can find more info in the Official Specifications Site.