All the models for methane are equally valid ways of representing the same thing; one kind of model may be preferred over another, depending on the level of detail at which you wish to visualize a molecule.
First, we see the model, a representation of lines connecting the center of each covalently-bonded atom.
A model represents bonds as lines and atoms as spheres, very similar to physical models that you may have used in organic chemistry, which give an impression of structure.
models represent the molecule as a composite of spheres, the radii of which have been chosen to approximate van der Waals contact distances. Spacefilling models are intended to portray overall molecular size and shape.