c++ function definition prefixes...(__stdcall)

I want a really detailed explaination for what is being said by attatch certain or any prefixes to the begining of functions (C++). For example:

__stdcall
 __cdecl

What are these doing to the function? I don't understand how they fit into the syntax of function definition. Don't they break the concept of <access> <return_type> <function_name>(<parameters>)?

Thanks,
Matthew