There are two kinds of function calls in Solidity: internal and external function calls. An internal function call is when a function calls another function in the same contract.

An external function call is when a function calls a function of another contract. Let's look at an example:

contract sample1 
{ 
    int a; 

    //"payable" is a built-in modifier 
    //This modifier is required if another contract is sending Ether while calling the method 
    function sample1(int b) payable 
    { 
        a = b; 
    } 

    function assign(int c)  
    { 
        a = c; 
    } 

    function makePayment(int d) payable 
    { 
        a = d; 
    } 
} 


contract sample2{ 

    function hello() 
    { 
    } 

    function sample2(address addressOfContract) 
    { 
        //send 12 wei while creating contract instance 
        sample1 s = (new sample1).value(12)(23); 

        s.makePayment(22); 

        //sending Ether also 
        s.makePayment.value(45)(12); 

        //specifying the amount of gas to use 
        s.makePayment.gas(895)(12); 

        //sending Ether and also specifying gas 
        s.makePayment.value(4).gas(900)(12); 

        //hello() is internal call whereas this.hello() is external call 
        this.hello(); 

        //pointing a contract that's already deployed 
        sample1 s2 = sample1(addressOfContract); 

        s2.makePayment(112); 

    } 
}