What organization structure would I need for my company in the short term and long term?

In a typical Organization Design class in a B-school, you will be taught a plethora of organizational structures. While it might look useless, all of them have a purpose. A small company can work with a flat structure better and as it grows bigger, the structure keeps on getting more hierarchical. There are various types of structures like functional, divisional, matrix etc. and you need to understand and figure out which structure suits your company best at which stage for its optimum functioning.