3D (three-dimensional) refers to any shape that has width, height, and depth. All 3D shapes are solids. Similarly, 2D (two-dimensional) refers to a shape that has width and height but no depth. You have to wear a special pair of glasses to see the 3D effect of a 3D film.
The difference between regular and 3D film is based on the illusion of depth shown for the objects in the film. For example, if you see a bird flying in a 3D film, you would have the illusion that the bird is flying in front of you and you can touch it, while in reality you cannot, as it is just a projection.