Time Dimension

Hi,
Please help me to understand about time table/dimension, I have below specific questions:
a) why we need to create that table/dimension?
b) Is this the builtin table in Management Studio or do I have to create one?
c) Why we need to create Time table in management studio (then create time dimension)?
I know it's very basic question but please send me some link which will help me to understand in deail.
Regards,
Amir