On what bases are solids, liquids, and gases differentiated?

If you consider the molecules of solids, liquids, and gases, you can see how they are in fact very different.

the molecules of a solid (such as ice) are stuck together in a rigid form.

The molecules of a liquid (water) are a little less stuck together, but they do have their boundaries.

The molecules of a gas (steam) have no boundaries, as steam does not have a definite volume. This means the molecules are free to roam around all over the place.