Sub Categories

Hi

I am new to word press. The blog i want to create is for a large organization with lots of departments within it. I wanted to know how i can create the illusion of multiple blogs while having only one. Each department name will be a category name from the blog home page. Once a department (category) is selected i want a new set of categories to be available within that sub-category.
Also, i would like somehow to duplicate the sub-category names across the various departments. I.e, i want category_A and category_B to both have sub categories called ‘operations’ but they are essentially to be two different categories.

Can i manipulate wordpress to make any of this possible?