Truck drivers are important when you think about it. They are responsible for doing inspections and keeping operations safe.
I believe it. How long does someone usually have to train to become one? 
They provide such important services like transporting goods across land. 
What sort of goods do they usually carry?
Items for retail businesses, raw materials, finished goods, etc.
This may be a little silly, by why are they called "semi" trucks?
Well, they are a combination of a tractor and any number of semi trailers. They load freight in them.