The term pore is used to describe the small openings in the skin in which oil and sweat reach the surface from their respective glands below. When you talk about “large pores” or “blocked pores,” you’re referring to oil pores, or hair follicles.
Hair follicles in fact have an important job. They allow the oil generated in the sebaceous glands to reach the surface and lubricate the skin. The skin’s natural oil, called sebum, helps keep skin supple and healthy.
Sometimes this process goes awry, a blockage develops in the hair follicle and acne blemishes appear. Acne itself is a disorder of the pilosebaceous unit, which is made up of the hair follicle, sebaceous glands and sebaceous duct.
Incidentally, sweat pores can become blocked too, although an acne blemish doesn’t form. Instead heat rash or “prickly heat” develops.