Form has to do with 3D (length, width, depth) shapes. Forms can be geometric (e.g. spheres, cubes, cones, cylinders) or free-form/organic (irregular forms such as mountains, trees, etc). Artists working with form often make sculptures, but I guess you can also represent 3D forms with 2D materials (e.g. drawing on a 2D piece of paper) because none of our form activities were actually 3D.
Form and value are closely linked – e.g. you can create form using value (shading).