Informal simply means non-official, relaxed, or friendly atmosphere consisting of vocabulary and grammatical structure of everyday language.
Communication is the exchange or imparting of information through media such as verbal or oral, or written.
Informal communication is an unofficial discussion taking place in the workplace between individuals. It can occur verbally/orally or in written form, not including any official responsibility.
Informal communication is also known as Grapevine communication, and the communication network is referred to as grapevine. Informal communication can also be with no words but only with gestures. Grapevine communication is independent of hierarchy which can also occur between an executive and a CEO. Informal communication is a great way of building a strong work relationship.