First difference is explained in terms of objectives of communication. Formal communication is a communication system which is to achieve the organizational objective. But the informal communication is the system to achieve individual objectives in an informal group. Every communication has certain objectives to fulfill. A formal communication is the communication which occurs in a formal way. Superior to subordinate communication or subordinate to superior is an example of formal communication. The formal communication is focused to do the tasks or jobs which one has to take. There by the organizational goals can be met. Informal communication meant to express the individual desires and interests. People find relaxed and comfort when they are in an informal gathering where they communicate each other their interests and tastes without any official barriers.
Second difference comes with the structural difference. Formal communication has a formal and well defined structure. Informal does not have a structure. There is a clear authority to report and get the instructions in a formal communication. This structure cannot be deviated as it ensures the smooth flow of communication and organization efficiency. Informal communication doesn't consider a structure as the people interact without any authority and level of job at which one works. A lower level employee may make a friendly talk with his superiors or high level officer without any official format.
The form and relationship of communication are discussed in the third difference. Only official and correct information are communicated in a formal communication. Informal communication takes both official as well as rumors. Information is officially communicated like the orders and instruction sent from the superior to sub ordinate. Informal communication takes the informal talks and gathering among the employees. Also relationships are controlled by rules, regulations, procedures etc. in formal communication. In an informal communication, the relationships are controlled by social and psychological needs of members.
Formal communication is in flexible nature. Informal is flexible. Once the duties and responsibilities are fixed, everything is to be communicated officially. These things cannot be changed frequently. Informal communication is flexible because it does not have the structure. The individuals in a group make the communication and they can be flexible as they like.