Business Process Flow-process template,Activities,Workspace

Hello

I was going thorough the creation of BPF and found few things doubtful:

1. Process Template: In the process template what is the role of 'Dimension' and why it is used?

2. Activities: role and importance  of 'Driving dimension'

Can some one clear my doubts with basic example or else share the link of any documents regarding the same.

Thanks

Red