Liquid Crystals as the name implies are materials that posess that share the properties of both crystals and liquids. Like some naturally occurring crystals, they possess an optical anisotropy that can affect the polarization state of light, as seen in the "polarizing microscope" image to the right (In this microscope optically isotropic materials such as water would appear black). But because of their liquid like nature, their optical properties can also be switched at very low voltages. Research in liquid crystal materials for more than 3 decades has led to the domination of LCDs in TVs, cell phones, calculators, wrist watches because of the various benefits they offer compared to prior CRT technology (portability and power consumption mainly).
Beyond LCDs, liquid crystals can be engineered to do more amazing things. By controlling the local alignment of these materials, many interesting optical elements such as gratings, lenses, and other complex patterns can be created.