Simply put large pores are just skin pores, which have become bigger than normal. This is usually as a result of over production of oil causing the pore to swell and enlarge. Large pores are usually associated with blackheads.
People with oily skin tend to suffer more with large pores as their pores can become blocked and enlarged. Ageing is also a contributing factor, particularly sun damage related ageing, as this can cause the pores to dilate and become larger. Loss of elasticity due to ageing can also make the pores look bigger. Hormonal changes also play a part and women may find they have more enlarged pores during their period. Genetics can also play a part.
Large pores generally appear on the face, particularly around the nose and on the cheeks.
Men and women can both suffer from large pores but men tend to have larger pores than women as their skin tends to produce more oil.