[Tutor] class question

[Dave S]

I guess Im one of those guys for whom classes do not come easy !

class test:
    a=1
    b=2

    def  __init__(self):
    self.c=5
    self.e=6

    def somedef(self):
    self.f=7
    self.g=8

In a class structure, you can use either a simple a=2 or self.a=2. OK 
Ive read the text but am still not clear.
The self.a=2 tags this variable to the genrated instance via self .... 
ok  ... and appears to be a global variable.
So what happens with plain a=2.

What I am asking is when sould I use a=2 & when self.a=2 ?

Thanks in advance
Dave