A mapping is basically an array that can be indexed on any type, not just integers. It can also be seen as a way of linking data (usually strings) together. It consists of a lot of index-data pairs which are linked together in such a way that map[index1] returns data1.
A mapping can be created in a way similar to arrays:
You can also set that data by writing map["five"]="good".
If you try to set an index in a mapping that isn't already present in the mapping it will be added as well.
mapping(string:string) map=(["five":"good", "ten":"excellent"]);