What are the differences between Formal and informal organization?

Formal organization 

Is a fixed set of rules of intra-organization procedures and structures. As such, it is usually set out in writing, with a language of rules that ostensibly leave little discretion for interpretation. In some societies and in some organization, such rules may be strictly followed; in others, they may be little more than an empty formalism. 

Informal organization 
Is the interlocking social structure that governs how people work together in practice. It is the aggregate of behaviors, interactions, norms, personal and professional connections through which work gets done and relationships are built among people who share a common organizational affiliation or cluster of affiliations. It consists of a dynamic set of personal relationships, social networks, communities of common interest, and emotional sources of motivation. The informal organization evolves organically and spontaneously in response to changes in the work environment, the flux of people through its porous boundaries, and the complex social dynamics of its members. 

Tended effectively, the informal organization complements the more explicit structures, plans, and processes of the formal organization: it can accelerate and enhance responses to unanticipated events, foster innovation, enable people to solve problems that require collaboration across boundaries, and create footpaths showing where the formal organization may someday need to pave a way.

Difference Between Formal and Informal Organization:

Formal Organization may be defined as such organization in which authority, responsibility and account-ability of each member are clearly defined. 
 On the other hand, informal organization may be defined as such organization in which authority, responsibility and account-ability of each member are not clearly defined. 
Objectives 
 The purpose or objective of establishing formal organization is to achieve Pre-determined goals of an enterprise. 
Whereas, the objective of establishing informal organization is just to fulfill the psychological and social needs of its members. 
Stability 
Formal organization is generally more stable in comparison to informal organization, because it is established according to proper rules and regulations for achieving certain goals. 
 On the other hand, informal organization is not as stable as formal organization, because it is established, in the lack of proper rules and regulations. 
Need 
Formal organization is gene-rally more needful to every organization, in comparison to informal organization due to stability. 
On the other hand, informal organization is not as essential to every organization as formal organization due to absence of proper suitability. 
Size 
Formal organization is generally too large in size, in comparison to informal organization. 
On the other hand, informal organization is generally too small, in comparison to formal organization. 
Establishment 
Formal organization is established on the basis of proper rule and regulation, where authority, responsibility and accountability of each member are clearly defined. 
On the other hand, informal organization is established spontaneously. It doesn't require any rule and regulation to emerge. 
Use of Organization charts 
Organization charts is needed to show formal organization. 
But there is no use of organization charts in informal organization. 

Types of Communication 
In formal organization generally written or formal communication takes place to express the orders, views and feedback etc. 
On the other hand, in informal organization generally oral or informal communication takes place to express the orders, views and feedback etc. 

Flow of authority 

In formal organization generally authority flow from top to bottom vertically over the subordinates and responsibility goes from bottom to top over superiors.
On the other hand, in informal organization everyone has equal rights and duties, so there is horizontal flow of authority in this organization. 

Leader 
 In formal organization gene-rally managers are the leader of organization and so, each and every activity is done under their guidance.
On the other hand, in informal organization generally leader is elected by the members and so, each and every activity is done by members under his guidance. 
Relationship 
In formal organization each and every members has impersonal or formal relationship with each-other due to bound in rules. 
On the other hand, in informal organization each and every member has informal or personal relationship with each-other due to equal rights. 
Scope 

 The scope of formal organization is broader, in comparison to informal organization. 
On the other hand, the scope of informal organization is narrow in comparison to formal organization. 
No. of members 
In Formal Organization there is long list of members in this organization for achieving certain goals of organization. 
But On the other hand, in informal organization there is short list of members due to lack of certain goal.