static strucutres

Static rule for user defined data type(struct, union)

• The static modifier for the struct limits the scope of visibility of the structure to the current translation unit (i.e. the file).
• even structure also the data type like integer. but the differnet between the int and struct is , int is  predefined data type and struct is the user defined data type.
• so the following rules will be applicable to the struct and any user define data type ( union)
• when applied to a variable declared inside a function, the value of that variable will be preserved between function calls.
• when applied to a variable declared outside a function, or to a function, the visibility of that variable or function is limited to the "translation unit" it's declared in - ie the file itself. 
• For variables this boils down to a kind of "locally visible global variable".

Reference:

http://stackoverflow.com/questions/7259830/why-and-when-to-use-static-structures-in-c-programming