You will be asked questions about certain faces of a block, so this page shows how each face will be labeled.  The top, front, and right faces are the ones you can see in the block below.  The left face is opposite the right face (on the back left side of the image) and is not visible.  The back face is opposite the front (on the back right side of the image) and is also not visible.  The bottom face is opposite the top face and generally is not visible; it is the face on which each block sits.