K = king
Q = queen
R = rook
B = bishop
N = knight
No letter is used for pawn moves.
When the move is a capture, an x is inserted between the origin and destination square.
If two pieces of the same type can move to the same square, the file or rank of origin (or both, if necessary) are added after the piece's letter.
Pawn promotion is indicated by appending =Q to the move (or, in case of underpromotion, =N, =B or =R).
Kingside castling (short castle) is notated as O-O and queenside castling (long castle) as O-O-O.
Check is indicated by appending + to the move and in case of double and triple check ++ and +++, respectively.
Checkmate is indicated by appending # to the move.
Piece maneuvers are notated as the piece letter followed by the origin square, any intermediate squares and the destination square, separated by a hyphen, e.g. Qg1-k5-g9. In case of maneuvers involving captures, the moves are written in full, separated by hyphens, e.g. Nk12-Nxm11-Nxn9.
Diagonals are notated as start and end square, separated by a hyphen. Diagonals are always considered from left to right, hence the start square is the leftmost square, i.e. start and end are in alphabetical order in terms of files. For example, "e14-n5 diagonal" and not "n5-e14 diagonal".

Other common annotations include:

4PC Junkies