In case of usual compounds e.g. cholesteryl nonanoate & p-azoxyanisole, the crystalline solid, when heated does not melt directly into liquid state but passes through an immediate change that it is called liquid crystal before changing to liquid state. This is as shown below.

Thus liquid crystals have some properties of solid like double refraction of light, ordered arrangement of molecules or atoms. They also have some properties of liquids like surface tension, flow viscosity, ordered arrangement of molecules or atoms.

Solid state $\begin{matrix} \rightarrow\\ \Delta \end{matrix}$ Liquid crystal $\begin{matrix} \rightarrow\\ \Delta \end{matrix}$ Liquid crystal

There are three types of liquid crystals

1)  Smectic (Soap like)

2)  Nematic (Thread like)

3)  Isotropic

1)  Isotropic –

They appear optically, magnetically electrically etc. to be same from any direction in space.

2) Nematic –

The Nematics liquid crystal phase is by for the most important phase for application. In nematics phase all molecules are aligned approx. parallel to each other.

3) Smectics –

In smectics, the molecular centers of mass are arranged in layers and the movement is mainly limited inside the layer.