The differences between a static member function and non-
static member functions are as follows.

A static member function can access only static member 
data, static member functions and data and functions 
outside the class. A non-static member function can access 
all of the above including the static data member.

A static member function can be called, even when a class 
is not instantiated, a non-static member function can be 
called only after instantiating the class as an object.

A static member function cannot be declared virtual, 
whereas a non-static member functions can be declared as 
virtual

A static member function cannot have access to the 'this' 
pointer of the class.
The static member functions are not used very frequently in 
programs. But nevertheless, they become useful whenever we 
need to have functions which are accessible even when the 
class is not instantiated.