A contract can have exactly one unnamed function called the fallback function. This function cannot have arguments and cannot return anything. It is executed on a call to the contract if none of the other functions match the given function identifier.

This function is also executed whenever the contract receives Ether without any function call; that is, the transaction sends Ether to the contracts and doesn't invoke any method. In such a context, there is usually very little gas available to the function call (to be precise, 2,300 gas), so it is important to make fallback functions as cheap as possible.

Contracts that receive Ether but do not define a fallback function throw an exception, sending back the Ether. So if you want your contract to receive Ether, you have to implement a fallback function.

Here is an example of a fallback function:

contract sample 
{ 
    function() payable  
    { 
        //keep a note of how much Ether has been sent by whom            
    }     
}