Answer:
Solids have a well defined shape and this -the shape- is "independent" of the space, in other words if we take a rock (solid) its shape would be the same (kind of) no matter in which container we put it. The volume is also well defined and easy to measure. We can use a "ruler" to measure the rock. On the other hand, liquids and gases shape will depend on the container (solid container) or their shape will very similar to the solid body containing them; however liquids are more sensible to mechanical forces as Gravity and therefore their shape will result in a combination of the container and the force of gravity (for example), hence if we put water in a cylindrical container but not enough to fully filled then the shape of the liquid will be cylindrical but because gravity it will occupy just the bottom of the container, while the gases are going to expand until cover the whole space of the container.
In short:
Solids: Well defined shape and volume "independent" of the environment
Liquids: Shape dependable of the container, hence not well defined. well define Volume
Gases: Shape and volume dependable of the container.