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.
