Thunderstorms start in their cumulus stage where the storm cloud grows. They then move into their mature stage where the storm is causing heavy precipitation. At this point the storm cloud is at its biggest. Finally they reach the dissipating stage where the storm begins to lose strength and fade away.
Tornadoes start as thunderstorms until warm rises. As the warm air rises it can begin to rotate forming a mesocyclone. If this continues the air will form a vortex and form a funnel cloud. If the funnel touches the ground...
Then it's a TORNADO
Much like tornadoes, Hurricanes start as thunderstorms. Then warm air begins to rise and clouds form a ring. This ring, known as an eye wall, will begin to spin and grow. At this point the hurricane is at full strength and will only lose strength once it reaches land.
Tornadoes are measured on the Fujita scale with F1 being the weakest and F5 being the strongest.
Hurricanes are measured on the Saffir Simpson scale from 1 being the smallest and weakest and 5 being the biggest and strongest.