A light bulb produces light from electricity. In addition to lighting a dark space, they can be used to show an electronic device is on, to direct traffic, for heat, and for many other purposes.
The most common base for A bulbs is the E26/E27 medium screw. Globe bulbs are more full and round in shape than A bulbs, but they're available in various sizes. These bulbs are used in bigger light fixtures like chandeliers and ornamental fixtures.