To be less blunt, a .TORRENT file contains values needed for a BitTorrent client to connect to other users so it can download and upload pieces. Each file is split up into very small pieces, and the .TORRENT file contains the checksum for each piece. A checksum is a virtually-unique value for a specific data chunk, in other words the checksum for the word "hey" can never be the same as the word "hello", or any other word for that matter. It uses the checksum to make sure that it has the *exact* same information as the original file, since if any of the information changed then the checksum would be different.

The .TORRENT file also contains the announce url, which is the location of the tracker. A user's BitTorrent client will tell the tracker "I need piece #213", and the tracker will give that user a list of people who have that piece. Once you finish downloading that piece, you tell the tracker "Ok, I have piece #213! Now I need piece #186". This will repeat until you have the entire file. The tracker will remember which pieces you say you have downloaded, and then when someone else asks for that piece then the tracker will tell that person that you have that piece, and they download that piece from you.

Among other things contained within the .TORRENT file, or what may be contained, is:
A list of alternate announce urls. (Optional)
A comment by the creator of the .TORRENT file (not necessarily the file your downloading). (Optional)
The date that the .TORRENT file was created. (Optional)
How many files are contained within the .TORRENT file.
If there are multiple files, then the location of those files.
The entire file's (or every file's) length.
The number of pieces.

There is more information, but most of it isn't directly related to the .TORRENT file itself. If your still curious then look at http://wiki.theory.org/index.php/BitTorrentFAQ for more information.