You'd need to parse the PGN files and extract information from that files in order to reconstruct a chess game. They are only "recordings" of chess games, if you're more confortable with that term. You COULD use PGN files to do that recording, or you could use your own programs to do that (as PGN files parsing is a more complex task). Use PGN files if you want to provide compatibility with the international chess leagues and such (it is a common format recognized by these authorities).

As already said before, you could make the game 100% server or client side based, or you could use both.

@dschreck: he asked about Portable Game Notation, not Portable Network Graphics (they have no generalities what so ever).

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