Both formal and informal communication are ways people interact, both on personal and professional levels. Typically, informal communication is not considered as reliable as formal communication, but makes day-to-day interactions easier.
Formal communication is that which utilizes a company's or organization's formal channels, such as memos, letters and emails. Formal communication leaves a written trail. In their personal lives, people use formal communication when sending letters of confirmation or complaint or when completing forms. Informal communication is that which is usually spoken, often off-hand, both in business and personal lives. It is this type of easy communication that allows people to share ideas and plans, and to accomplish those things in a timely manner.