Why do some aircraft carriers have an upward starting runway?
The upward slope on the runway is also known as a "ski jump", and it is to give VSTOL (Vertical or Short Take Off and Landing) aircraft get aloft on the short carrier deck. Carriers without this use aircraft catapults to quickly get airplanes up to flying speed. 

The advantages of the ski-jump are its simplicity, which means the carrier has lower build and operating costs. The drawbacks are that VSTOL airplanes are generally more limited in their capabilities, the airplanes that are launched by catapults pack a lot more punch.