confusion controllers vs subcontrollers

i am just starting to use bsp development, and have been going through the tutorials, etc. i have a question regarding controllers. i have seen several references to sub-controllers, and i am curious as to what makes a controller a controller versus a sub controller...

also, should an applicaiton have a controller for each view, or should one controller handle multiple/all views? i see a lot of conflicting information about this.

i just want to get started on the right path....