Guard cells are the two cells that enclose stomatal pore and so comprise the stoma. Their chracteristic structure of cell walls allow them to shift their shape depending on their turgor.
They control water flow through the plant but also gas exchange – they allow carbon dioxide to enter the plant and excess oxigen to leave it.
Guard cells alter in the plant kingdom. Dicots, many monocots, some mosses, ferns and gymnosperms (Pinophyta) have kidney-shaped guard cells, while grasses, palms ant some other monocots have dumbbell-like guard cell accompanied by other specialised epidermal cells, namely subsidiary cells. In plants that have those subsidiary cells the whole stomata is called the stomatal complex (in Polish it’s "aparat szparkowy" :D).
I hope it’s efficient description for now. As far as you’re not very acquainted with that topic, the physiological mechanisms of opening and closure of guard cells are not very required, aren’t they?

I always wonder how they managed to identify the whole process of ABA (abscisic acid)’s influence over the positioning of stomatal cells. I mean how the hormone sends the message carried by the intracellular messengers to the point of action. There are many stages and many proteins,ase, mRNAs and other chemical compounds. I don’t know how they identify the functioning of each chemical at each stage. And how many years it will take.