A mapping data type is a hash table. Mappings can only live in storage, not in memory. Therefore, they are declared only as state variables. A mapping can be thought of as consisting of key/value pairs. The key is not actually stored; instead, the keccak256 hash of the key is used to look up for the value. Mappings don't have a length. Mappings cannot be assigned to another mapping.

Here is an example of how to create and use a mapping:

contract sample{ 
    mapping (int => string) myMap; 

    function sample(int key, string value){ 
        myMap[key] = value; 

        //myMap2 is a reference to myMap 
        mapping (int => string) myMap2 = myMap; 
    } 
}