Stomata in plants are openings located on the surface of leaves. Stomata is the plural of stoma, a word that comes from Greek, and means "mouth". I found a great picture of a stoma in Wikipedia, which you will enjoy. Please look at it here.

If you go to this site, you will see more pictures of stomata. The stomata make possible the gasses exchange for the leaves; air enters the plant through these openings by diffusion.

Plants take in carbon dioxide from the air and, together with water, convert it into oxygen and sugar. The process is called photosynthesis. Carbon dioxide (and oxygen) cannot pass through the waxy layer covering the leaf but it can enter the leaf through the stomata. Oxygen, produced during photosynthesis, can also only pass out of the leaf through the opened stomata.