In a solid, atoms are packed very closely together, almost like an organized grid. They vibrate, and rub against each other, but stay in the same place.
In a liquid, atoms are farther apart, and slide around each other, which is why if you pour water on the ground, it won't stay in any specific shape unless it is in a container. However it does stay together. Gas on the other hand is very spread out with a lot of space in between the individual atoms. All the space allows gases to expand and contract. Gas atoms bounce around, and will not hold a form or stay together.