2D refers to objects or images that show only two dimensions; 3D refers to those that show three dimensions. Because reality exists in three physical dimensions, 2D objects do not exist. However, they can be portrayed in images and art.
2D and 3D generally refer to paintings, drawings and computer-generated images. While nothing real truly lacks a third dimensions, items can be depicted without showing a third dimension. The terms are sometimes used to determine whether a depiction of something is presented with clear signs of a third dimension. A drawing with a shadow, for example, might be said to be in 3D.
However, the term is sometimes used to refer to the medium of the depiction. Paintings, photographs and computer images are in 2D by this definition, regardless of what they depict. Sculptures, on the other hand, are in 3D. This distinction is common in the art world.
Further complicating matters are 3D films and drawings. By sending different images to each of the eyes of the viewer, images can simulate the third dimension. These depictions can be accurate, and the effect is enough to fool the brain. However, the depth dimension is only simulated, so some claim that these images are still 2D.