PGN and FEN are computer data formats describing a chess game or a chess position.
It helps to store games for later use, and, to move games to another computer

PGN

FEN

PGN file

On www.newbritainchessclub.com

1. find a board in starting position
2. make your moves (after a mistake you can take back a move)
3. select "showPGN"

On www.chess.com

1. select "Learn"
2. select "Analyze"
3. make your moves (after a mistake, hit "+" to start over)
4. select "Share" (bottom right corner)

Difficult to add variations...

On www.lichess.com

Collections on lichess.com