A bulb is a part of a plant that is usually below the ground, or partly below the ground. Bulbs are usually quite spherical but pointed at the top like a teardrop. Bulbs grow roots from the bottom and leaves from the top. Some types of plants that grow from bulbs are onions, lilies and tulips. Anything that has a similar shape to a bulb may also be called a bulb, such as the bulb at the bottom of an old fashioned thermometer.
A light bulb is a device that converts electricity into light, so you can see when it is dark. The picture is of an incandescent light bulb, which works by running electricity through a filament, which glows white hot. Other kinds of light bulb work in different ways, but they all convert electricity to light.