Informal organisation emerges from within the formal organisation when people interact beyond their officially defined roles. When people have frequent contacts they cannot be forced into a rigid formal structure. 

The formal organisation is a system of well-defined jobs, each bearing a definite measure of authority, responsibility and accountability

Delegation of authority merely means the granting of authority to subordinates to operate within prescribed limits.

Decentralisation refers to systematic effort to delegate to the lowest level all authority except that which can be exercised at central points.