The using A for B; directive can be used to attach library functions (from the library A to any type B). These functions will receive the object they are called on as their first parameter.

The effect of using A for *; is that the functions from the library A are attached to all types.

Here is an example to demonstrate for:

library math 
{ 
    struct myStruct1 { 
        int a; 
    } 

    struct myStruct2 { 
        int a; 
    } 

    //Here we have to make 's' location storage so that we get a reference.  
    //Otherwise addInt will end up accessing/modifying a different instance of myStruct1 than the one on which its invoked 
    function addInt(myStruct1 storage s, int b) returns (int c) 
    { 
        return s.a + b; 
    } 

    function subInt(myStruct2 storage s, int b) returns (int c) 
    { 
        return s.a + b; 
    } 
} 

contract sample 
{ 
    //"*" attaches the functions to all the structs 
    using math for *; 
    math.myStruct1 s1; 
    math.myStruct2 s2; 

    function sample() 
    { 
        s1 = math.myStruct1(9); 
        s2 = math.myStruct2(9); 

        s1.addInt(2); 

        //compiler error as the first parameter of addInt is of type myStruct1 so addInt is not attached to myStruct2 
        s2.addInt(1); 
    } 
}