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);
...