Informal communication, as opposed to formal communication, is described as conversations inside a firm that are not bound by a predefined framework. Consider these encounters to be a brief chat at the coffee machine, a passing remark with another employee, or an impromptu brainstorming session across teams.
When orienting myself with colleagues in the workplace, I would use both formal and informal communication because some aspects of my personality can be explained formally to establish boundaries, while informally making friends and mingling well with each individual.