Assigning multiple values using tuple

[sivacg]

As a newbie, I am trying to understand the assigning multiple values using tuples. May I request one of you to help me here?

>>> a=('x','y','z')
>>> a
('x', 'y', 'z')
>>> a=(1,2,3)
>>> a
(1, 2, 3)
>>> a=(u,v,z)
Traceback (most recent call last):
  File "<pyshell#26>", line 1, in <module>
    a=(u,v,z)
NameError: name 'u' is not defined

2nd piece:

>>> a
(1, 2, 3)
>>> (u,v,z)=a
>>> u
1
>>> v
2
>>> z
3
>>> a
(1, 2, 3)

What is happening here exactly?