Difference between ‘cls’ and ‘self’ in Python classes?

Why is cls sometimes used instead of self as an argument in Python classes?

For example:

class Person:
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname

    @classmethod
    def from_fullname(cls, fullname):
        cls.firstname, cls.lastname = fullname.split(' ', 1)