A 16x16 texture pack is a zipped collection of files that changes the texture of blocks, items, mobs and the GUI in the PC version of Minecraft's original resolution, 16 pixels high by 16 pixels wide. These files are typically PNG images. Resolutions above 16x16 are considered HD.
These texture packs have names such as Bluebird and Plediacraft. They alter the appearance of the traditional Minecraft setting by making the blocks and other objects change colors and appear to have texture, hence the name texture pack. The texture packs are created to adhere to multiple surfaces within the game and are available for use from a number of websites dedicated to the packs.