Although we're not covered head to toe in a suit of fur, our skin is abundant with hair follicles, tiny shafts through which hair can grow and reach the skin.
"Follicles" and "pores" are sometimes used interchangeably, and other times referred to as two different things. In truth, the pore is simply the opening upon the skin of the hair follicle, which extends downward through several layers of skin.
If a hair follicle were a tall chimney, the pore would be the opening at the top of the chimney. Instead of emitting smoke, the follicle emits a shaft of hair. Skin cells are constantly dying inside the follicle. Additionally, small sebaceous glands located inside the follicle (picture a cul-de-sac located off to the side of an otherwise straight road) produce oil called sebum. Sebum is a mixture of fats, proteins, cholesterol and inorganic salts. It travels up the follicle and (in a perfect world) exits through the pore. It also carries those dead skin cells found within the follicle up to the skin's surface.
What about sweat -- doesn't that come out of the same pores? No. Sweat is produced by separate sweat glands that also heavily populate your skin. While sweat emerges from the skin from a different source, it does affect your skin's appearance. Once that sweat reaches the surface, it dries but leaves salts behind that can block your pores.
This mix of oil and dead skin cells helps coat your skin to protect it from bacteria, viruses, wind and rain (we sort of take our skin's protective qualities for granted). Sometimes, though, the pore is occluded (blocked) and the materials trying to get out can't, resulting in acne.
For more information about skin cells, read Skin Cells: Fast Facts.
If you have large pores, there's some good news -- but you may have to wait a few years to receive its benefits. As we age, our skin produces less oil, leading to dryness. This dryness, coupled with environmental damage to the skin, causes skin to age and wrinkle. Large pores produce more oil, and this comes in handy later in life when your skin needs it most. So while you may be bugged by the appearance and size of your skin pores today, you're just getting an early start -- and their existence will please you down the road.
Next: Maintaining clean, open pores.