Stomata act as the 'pores' on the 'skin' of plants, allowing for gas exchange.
Open stomata allow carbon dioxide to pass into the plant, and for oxygen to be expelled. In this way, stomata provide the mechanism for the plant's respiration.
The stomata also allow water molecules to escape from the plant. This process is called transpiration. This process is detrimental to the plant: it can become dehydrated. However, the importance of the gas exchange (and the photosynthesis that occurs as a result) outweighs the negatives of transpiration.
The size, and status of each stoma (either open or closed) is controlled by gaurd cells (parenchyma).
The shape, number and density of stomata varies by plant species. Grasses, for example, usually have a lower concentration of stomata than deciduous trees. The stoma of grasses are also dumbbell shaped, whereas tree stoma are shaped like kidney beans.