Two Dimensional Art (2D): Art that is flat. Painting, drawing, and photography are considered 2d art. Artist may "trick your eye" by manipulating values to make a 2d work of art look 3d.
Three Dimensional Art (3D): Art that has height, width, and depth (thickness and deep space). Art that is not flat. Sculpture and architecture are considered 3 dimensional works of art.
Relief: Has 2D and 3D qualities. A relief is something that has 3 dimensional qualities protruding from a 2 dimensional background. Some sculpture is considered relief sculpture. Many pieces of architecture has pieces of relief art worked into the architecture.
Test your knowledge: Which work of art is 2d, which is 3d, and which one is a relief?