Solids, liquids, and gases have very contrasting properties. These contrasting properties are explained by the underlying, particle-level characteristics. One of the most important particle-level characteristics is the intermolecular forces acting between adjacent particles. These intermolecular forces are strongest in solids and weakest in gases.
The solid state of most materials can be described as a condensed state in which the forces between particles pull the particles close together and into a locked position. The particles are held in this position and incapable of moving throughout the bulk of the sample. They can only vibrate about this fixed position. The particles commonly organize themselves into an internal structure, exhibiting a regular and repeating arrangement of particles throughout the bulk of the sample. No matter what container the solid is placed in, its shape and volume will not change since the intermolecular forces hold the particles of the sample together.
The gaseous state lies on the opposite end of the state continuum. The intermolecular forces in the gaseous state are so weak that they have negligible influence upon the particles in the sample. Particles are thus capable of moving throughout the bulk of the sample. The extreme to which they can move is defined by the walls of the container that holds them. They do not form any organized pattern or arrangement but are instead in a constant random motion at high speeds. They are as spread apart as they can be as they simply fill the container that they are in.
The liquid state can be thought of as the intermediate state between solids and gases. The intermolecular forces between particles in the liquid state are weaker than that of the solid state but stronger than of the gaseous state. While the forces are not strong enough to hold the particles of the sample in a locked position with an ordered arrangement, they are strong enough to result in a condensed state with particles existing in close proximity to one another. But unlike solids, the particles in the liquid state are able to slide past one another and exhibit some particle movement in addition to the usual vibration observed in solids. Because of the weaker intermolecular forces, a liquid will have a variable shape that is influenced by the container that it is in; this is in contrast to the fixed shape of solids. However, the forces in the liquid state are strong enough to prevent the particles from filling the entire container. Thus, liquids are said to have a fixed volume; this is in contrast to gases that have a variable volume that is governed by the container that the gas is in.