PGN (Portable Game Notation) is a widely-used format that records a chess game’s moves in standard algebraic notation along with related data such as player names, the result, and the game date.
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Here is an example of a PGN taken directly from the chess.com archives:

When you copy a PGN block and post it in a forum as a Game or Sequence of Moves diagram, readers can play through the game using the navigation arrows and view any comments you've added. The diagram will also display game details like player names and dates at the top.

If you prefer, you can copy just the moves (e.g., 1.e4 through 16.exd3), but this won’t include any additional game details unless you manually enter them in the 'Game Details' tab of the editor.

FEN (Forsyth-Edwards Notation) is a concise way to describe a single position on a chessboard. It doesn't provide information about the moves leading up to that position, so it can’t indicate potential draws by threefold repetition, for example. However, FEN does include important details such as whose turn it is, if there's a chance for castling, and the number of moves played.

Example of FEN strings:

FEN strings are particularly useful for showing a single position or if you want to post a puzzle. To do this, simply paste the FEN into the Chess.com Diagram Editor, enter the solution moves, and insert it. This method is easier than manually placing each piece.

To get an FEN string from a game, go to the bottom of the game screen and click the

icon. Then select the PGN tab. The FEN will be listed above the PGN: