What are hash files?
  Hash files are certain files from a Battle.net game that are used to check that the game is correct, up to date, and unedited. They are used in a system which is called CheckRevision as part of the logon sequence of Battle.net.

Where can I get them?
  If you have the game you need hash files for installed on your computer already, and it's updated to the current version, you can get the hash files from the game folder itself. If you don't have the game you wish to use, you can get hash files from the links listed below.

What about verbyte values?
  The Version Byte value (verbyte) is a hexadecimal numeric value, thus far a single byte, representing the current version of the game. As it changes with every major revision (and some certain sub-revisions of certain products), you'll probably need to update your verbyte as well as your hash files. The verbyte value can be found by hovering over the current game version. It is listed in Hex, preceeded by an 0x identifier, and is followed by the decimal notation value. Your client may need one or the other in order to connect correctly.

Common Mistakes
  Hash files will not update your game if you replace an older copy with these files. Doing so will corrupt your game and require you to replace the files or reinstall. These files will not allow you to play games, and can not be run independently. They are simply for handling CheckRevision.

Legalities
  Blizzard Entertainment owns the rights to these files, and downloading them may be unlawful. Since they are of no use on their own, and can not be used to obtain a full copy of any Blizzard game, I honestly don't see why it matters. I will continue to make these files available by whatever means I may for the convenience of my users.