The defining characteristics between the phases of matter are the kinetic energy of the molecules and the density of the particles to one another.
Take water for example. Below 0°C water is a solid, ice. Above 0°C but below 100°C,
you will have liquid water. When water is heated above 100°C, it becomes a gas.
In ice, solid water, the molecules are so tightly packed they are fixed in a rigid structure.
As a liquid, water molecules can slide past one another in a smooth motion, but they remain touching. This is why water will conform to the shape of its container but still keeps a constant volume.
Water vapor has high kinetic energy so the molecules move very quickly. There is a great deal of space between the particles so they shoot off in every direction, spreading farther and farther apart until they fill the entire room.