The important characteristics of the three states are listed out below:
Solid state
A solid possesses a definite size (volume) and a definite shape under ordinary conditions; and tends to maintain these even under deforming conditions. The substances in solids are closely packed and bound by strong inter particle attraction, making them rigid and geometrical. Some common examples of solids are iron, silver, common salt, etc.
Liquid state
A liquid possesses a definite volume but not definite shape. The substances in liquids have particles, which are loosely packed and bound to each other by forces weaker than those of solids. This makes a liquid mobile and shapeless resulting in its taking up the shape of the container in which it is placed. A liquid also has a tendency to flow. For example, water, alcohol, milk, oil, etc.
Gaseous state
A gas neither possesses a definite volume nor a definite shape. The substances in a gas have particles that are separated by great distance, having virtually no force of attraction between them. A gas occupies the whole of the volume of the vessel in which it is placed. It also takes up the shape of the container. For example, air, carbon dioxide, oxygen, hydrogen, etc.
The three states of matter are inter-convertible. This can be done by heating or cooling. Heating increases the inter particle spacing and kinetic energy of the particles. So, a solid on heating gets converted into a liquid, and a liquid into a gas.
