Different types of textures. Textures are multidimensional blocks of GPU memory, and the texture's type determines how many dimensions there are, and adds some semantics about what the 3rd dimension means.

2dA single 2D image, the most common type.
3dA 3D image, where a sequence of 2D images defines a 3D volume. Each mipmap level of a 3D texture gets smaller in the x, y, and z axes, unlike cubemap and array textures.
cubeSix square 2D images with the same dimensions that define the faces of a cubemap, used for skyboxes or other "directional" images.
arrayArray textures are sequences of distinct 2D images that all have the same dimensions.

See also