A pore, in general, is some form of opening, usually very small. Pores can be found on many organisms, such as in plants, animals, and humans.
More commonly, in talking about the skin, a pore is an opening into a sebaceous gland that secretes oil to lubricate and protect the surface of the skin.
Enlarged pores can be a cosmetic problem, especially during puberty, where pores, especially around the nose (the "T-zone") enlarge in order to cope with increased oil output. Often, after puberty ends, and the oil glands approach a regular output, the pores may consequently decrease in size too.
In plants, pores exist in leaves in order to aid in photosynthesis and respiration. In these cases, the pores are known as stoma. Many organic substances are porous (containing many pores), and then can be very light. Such substances include wood, and the bones of some birds, which porous structure enable these substances to be very light, but strong.
Cells often contains pores in order to facilitate the movement of water in and out of cells by means of osmosis.
In geology, pores refer to the open space within a rock. Just as a stack of oranges in the grocery store have spaces between them, so do the grains of sand in a sedimentary rock. Pores in rock are where groundwater, oil, and natural gas are found.