Stomata are present on the lower epidermis(underside) of leaves. There are few, if any, present on the stem or upper epidermis. Stomata are extremely tiny, and they are present in lange numbers - about 200 per square mm.
The main function of stomata is to allow gas exchange to allow the plant's cells to respire and photosythesize. Stomata are bordered by two kidney shaped guard cells, which regulate the size of the opening, partially closing it during the night, where carbon dioxide is not needed for photosynthesis, and during times of drought, where the water vapour lost through the stomata is too great.
The guard cells have a thick inner wall(facing the opening) and a thin outer wall. This is so that when the pressure inside the cell increses, the cell will bend due to the differing stregnths of the wall, and the stomata will open.
So how does the pressure inside the guard cell change in response to the light present? It's a complex process. When light is hitting the cell, photosynthesis is taking place in the chloroplasts, which produce energy (ATP). In the cell membrane of the guard cells there are proton pumps, which, if ATP is present, pump out protons (H+ ions). This creates an electrochemical gradient, so K+ ions diffuse into the cell. The lack of H+ ions also creates a rise in pH, so Cl- ions diffuse in to counter this. This whole process leads to an decrease in the water potential(ψ) inside the cell, so water enters the cell through osmosis, increasing the pressure (=making it turgid) and thus opening the stomata. In darkness, the reverse occurs, with H+ ions diffusing back in and K+ and Cl- ions diffusing out, leading to increased water potential, lower pressure, and a narrower stomatal aperture. Note that the stomata never close completely.