There are three stages to the life cycle of a thunderstorm and each is defined primarily by the movement of air within the cloud.
At this stage, all the air movement within the cloud is upwards. The tops and sides of the cumulus are growing and large water droplets are suspended in the updraughts. Some rain or virga may be present.
The cloud is bright white and the edges are sharply defined. The surrounding air is being sucked into the cloud from below and the sides.
Water droplets have grown larger and rain starts falling from the cloud base. Downdraughts form and the unstable nature of the cloud can cause the downdraughts to be very strong. There are now both updraughts and downdraughts within the cloud, creating strong wind shear and severe turbulence as the two interact.
Lightning and hail are common in the mature stage.
The top of the cloud often reaches the top of the troposphere, where the cloud top then spreads out into the classic anvil shape.
The dissipating stage starts when the updraughts have stopped. The whole cloud is now colder than the surrounding air, so it slowly begins to subside and warm. This can create some strong downdraughts at first, but as the overall temperature of the cell approaches the environment temperature the downdraughts weaken. The cloud starts to evaporate and the edges appear ragged.
A great timelapse video of a thunderstorm forming can be seen on YouTube here: https://youtu.be/232LFz-aiz4