3D is the short form for three dimensional space. In web applications, it refers to an image that can be rotated to view the whole objects from different camera angles.
3D refers to three dimensional space, which basically means all the physical objects that we see and touch in our everyday lives. In web applications, 3D refers to imagery that can be rotated to see the object in different camera angles. Normal text that we see (like this site) is in 2D.
Many examples of 3D imagery is available on the Internet. Google Maps Street View is a good example that shows a panorama of the street in 3D.