Stomata is just the plural form of 'stoma', which are the tiny pores on the underside of the leaf. Guard cells control whether they open or close, and therefore also control the exchanging of carbon dioxide and oxygen. I'm pretty sure I already covered how they work in another post, but I'll just write it again ... (bear with me)
If there are a lot of potassium ions present in the fluid in the guard cells, the cells will absorb a lot of water via osmosis, and will become more turgid. This opens the stoma, allowing diffusion to take place. If there aren't a lot of potassium ions present, the cells will lose water, become flaccid and close the stoma. The opening and closing of the stoma also depends on whether it is day or night. In the light, the guard cells absorb water, in the dark they lose it.
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