newbie question on .pgn files

Please excuse the silliness of this question, but what does one do with a .pgn file? Is that just the format that particular chess engines use to analyze games?

The only chess engine, if I am correct in calling it that, that I use at the moment is the one on this site.  Should I be looking into another if I want to study/train more efficiently.

Or are there other advantages of .pgn files of which I am unaware?

Oh, wait . . . pardon me for thinking alound (this is kind of one of my problems as a chess player, not planning what I am going to do well enough), but are .pgn files how people include their games in these forums for analysis?  

What else am I missing.

Thanks so much for anyone who has been patient enough to read this, and is willing to answer such a newbie question.  

If you want to write a chess game into a file so that an engine, or a chess database program, ... can read it and work with it, then you write it into a so-called pgn file.

(A pgn can also contain just part of a chess game, with a starting position and some of the following moves.)

For example, you can write there the players' names, the date and year when the game was played, the moves of the game in chess notation, the result, ... just all of the info that is most useful about a chess game. In a pgn file, all this info is contained in a format that will be readable by the most of the common computer programs used for chess.

And yes, if you have a game in pgn format on your computer, and if you wish to share it for example in a diagram in a chess.com forum comment, you can just copypaste the pgn file's content into that diagram when making the diagram. (You can also choose to enter the moves graphically in that diagram, the pgn will be automatically generated from that info then.)

PGN = "Portable Game Notation" is a universal standard for storing Chess games in machine-readable form (e.g. on files, web-pages, the clipboard for copy-pasting). Virtually every computer application that has anything to do with Chess will accept it as input format, and will be able to generate it as output format.

This includes Graphical User Interfaces like WinBoard, Fritz or Shredder, database programs like SCID or Chessbase, etc. Database programs usually have a separate private format for storing the games in compressed form, because PGN, being based on human-readable text, is not very compact. And this starts to hurt when you would have a file with a few million games on it. So the programs will be able to read or export PGN, but then convert it to their internal format (which to humans would look like gibberish) for more efficient processing on future reference.

For clarification only, here is what a PGN file looks like:

[Event "Thematic Tournament"]
[Site "Chess.com"]
[Date "2011.11.16"]
[White "TenaciousE"]
[Black "Anonymous"]
[Result "1-0"]
[WhiteElo "1847"]
[BlackElo "1513"]
[TimeControl "1 in 3 days"]
[Termination "TenaciousE won by resignation"]

1.d4 d5 2.e4 {Thematic Game - This is the starting position.}
 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3 e6 6.Bg5 h6 7.Bxf6 Qxf6 8.Bb5+ Bd7 9.O-O Bxb5 10.Nxb5 Bd6
 11.Ne5 a6 12.Nxd6+ cxd6 13.Rxf6 gxf6 14.Ng4 f5 15.Ne3 d5 16.Qe2 Nc6 17.Nxd5 Kd8 18.Nb6 Rb8 19.c3 Ne7 20.Qe5
 1-0

Formatting is standardized (header info, comments in 'squiggly brackets', etc.). 

See: http://en.wikipedia.org/wiki/Portable_Game_Notation

Hey, that is all really incredibly helpful.  Thanks so much.  I really appreciate the example as well.

.PGN stands for Portable Gamming Network used to store chess-game in basic text format. U can see info of .pgn using simple text editor like notepad

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.PNG is used for Images(as like .jpeg and .gif) and .PGN for games.