Communication system transmits information from one place to another place separated by a distance, sometimes a few meters and sometimes thousands of kilometers. Communication between two computers in the same room is a good example for short distance communication while telephone conversation between two people in two continents for example in Asia and Americas is a good example for long distance (in fact ultra long or transoceanic) communication.
Information is carried by an electromagnetic carrier wave whose frequency can vary from a few megahertz to several hundred terahertz. Optical communication systems make use of high carrier frequencies, over 100 THz (Terahertz) in the visible or near infrared region of the electromagnetic spectrum. To differentiate optical systems from microwave systems, such high frequency systems are called Lightwave systems.
Lightwave systems typically offers Gigahertz frequency for carrying information and hence transmission capacities are expressed in Gigabits. Light wave systems can be directly employed as in free space optics or through wires often made of glass or plastic as in fiber optics.