If Minecraft is already running, make sure to leave the world. However this is not necessary, as of snapshot 12w23a, for as long as pack.txt exists, it is recognized as a texture pack. In-depth instructions on obtaining the files to make custom texture packs are located at Tutorials/Custom texture packs. Most texture packs are in ZIP file format, but as long as it has the necessary files ( pack.mcmeta), Minecraft recognizes a folder as a texture pack.
The root directory also contains an optional image called pack.png, which shows as the thumbnail for the pack on the texture pack selection menu. A texture pack is identified by Minecraft based on the presence of the folder of the root directory, which contain a text file called pack.mcmeta, which would contain a description of the texture pack which would appear in game.
