Texture packs are activated in the same manner as any of the game's resource packs. The procedure differs slightly depending on whether you are playing through Windows, Linux or Macintosh OS X.
Texture packs should be in .zip file format when downloaded. Windows users should move this file to the subdirectory %appdata%/.minecraft/resourcepacks. Mac OS X users find the correct subdirectory at /Library/Application Support/minecraft/resourcepacks. For Linux users, the correct directory to which to move the file is /.minecraft/resourcepacks/.
Once the file has been moved, start the game. Select "options" in the menu, then "resource packs." The new texture should appear in the list of resources. Click on it, then click "activate" to make it available in the game.