Have you ever found yourself looking at someone else's function and wondered what the argument *args, **kw, or **kwargs means / does?
Example:

def my_function(proto, *args, **kw):
    # rest of the function code here

Those arguments are called "Keyword Arguments". Essentially, they are place holders for multiple arguments, and they are extremely useful especially when you need to pass a different number of arguments each time you call the function.
Example:

>> my_function('filter', 'python', 'html', start='now')
>> my_function('proto', 'filter')
>> my_function('ask', pg='2')

args, kw, and kwargs themselves aren't "special", it's the asterisks (*) that makes it "special". One * will create a tuple out of all the one off arguments (e.g. 'filter'), where as the double ** will create a dictionary out of all the arguments that have an equals (e.g. start = 'now').
note: If you're using both the single (*) and double (**) asterisk in your function, the single (*) MUST come before the double.

A Simple Test

def kwargs(basic, *huh, **no_way):
    print basic, huh, no_way

>> kwargs('proto', 'filter', '.com', a='pythonic', maybe='yes')
>> proto ('filter', '.com') {'a': 'pythonic', 'maybe': 'yes'}

>> kwargs('girls', yes='no')
>> girls () {'yes': 'no'}

So, if you don't want your functions to raise an TypeError or you need your functions to accept variable number of arguments, you can use "Keyword Arguments".