Whitish to bluish microscopic arachnids. They have eight legs and are slightly hairy. I could go on, but you might just want to look at the pictures.
Single Dust Mite
Dust Mite Up Close
Dust Mite Infestation

Amid the other components of dust (dirt, dried feces, skin cells, pollen, dander, spider webs, mold and fungus spores, hair, etc.) are microscopic arachnids called house dust mites (not dustmites). There are two types commonly found in homes: the European house dust mite (not dustmite) and the American house dust mite. Both can be found in practically every home in North America, and both are placed in the genus Dermatophagoides. Their close proximity to humans is due to the fact that these tiny little whitish to bluish arachnids' favorite meal is sloughed skin cells. Since the average person sloughs roughly 12 grams of skin cells a day, it makes sense. Aside from skin, dust mites can and do eat, among other things, mold and fungal spores, pollen, feathers, wool, and dried semen.
Remove surfaces that are conducive to high dust mite populations. If you don't want them in your home, don't make them comfortable. First and foremost, the best thing you can do for dust mite allergy control is to get rid of all wall-to-wall carpeting. There is nothing that makes dust mites happier than carpeting. Upholstered furniture should be next on the list, especially the big, fluffy, overstuffed stuff. If you can afford to, replace it with leather or vinyl. Don't use bed skirts or bed canopies. Replace heavy drapes with washable curtains or easily cleanable blinds.
Clean house! For getting rid of dust mites you need to be dusting at least once, preferably twice, a week. Use a microfiber cloth and start high, like ceiling fan blades, tops of picture frames, and tall shelves, and work your way down. Once you're done dusting, immediately vacuum both the furniture and the floors. After vacuuming, start mopping. Many people feel compelled to sweep before mopping. If you're trying to remove dust mites, this is a no-no. Sweeping causes things that have settled to the floor to get kicked back up into the air. Before doing any dusting, vacuuming, etc., make sure to eliminate all drafts. Close doors and windows and turn off fans and air conditioners.
In the bedroom. Since so much time is spent in the bedroom, you should put forth a little extra effort when it comes time to eliminate dust mites found there. Wash sheets and pillowcases once a week in hot water. The hot water will kill dust mites and dust mite eggs and wash away most of the old nasty skin cells that you've dropped in them. If you use down comforters, wool blankets, or feather pillows, swap them out. Use bedding made from cotton or synthetic fibers. They're not as tasty. Finally, pick your crap up. Keep your clothes in the closet with the door shut or in the dresser.
Temperature and humidity. Dust mites thrive in warm temps and high humidity. Set thermostats to below 70ºF and do everything you can to reduce humidity. This is important because dust mites don't drink water. They absorb it from the air. Set up some dehumidifiers, install fans in the bathroom and above the stove, fix leaky faucets and pipes, insulate pipes to avoid condensation, get rid of aquariums, install fans in rooms with poor circulation, and keep the doors to those rooms open. When it comes time to mop, forgo the damp mop, spray a towel with your cleaner of choice, get on your hands and knees, and scrub. While a relative humidity of less than 70 percent is helpful, you should aim for around 35 percent.
Use HEPA air filters along with a HEPA air purifier. The words corresponding to the acronym HEPA are "high efficiency particulate air." These are the most efficient filters available for dust mite removal. HEPA filters remove 99.97 percent of particulate matter from air that passes through them. This includes dust mites, dust mite feces, and dust mite remains. HEPA filters are available for furnaces, air conditioners, and central air units. You should also use a HEPA air purifier. HEPA purifiers look and work much like fans. The main difference is that when air is sucked into the unit, it passes through a HEPA air filter before being released back into the wild. Also consider purchasing a HEPA vacuum, a HEPA filter for your current bagless vacuum, or a microfiltration bag for your bagged vacuum.
Dust mite‒proof bedding. It's highly recommended for people learning how to get rid of dust mites to invest in some dust mite mattress covers and some dust mite pillow covers. The brand of dust mite-proof mattress covers that I've been using for a couple years now is Protect-A-Bed. Regardless of the brand of dust mite covers you decide to go with, the benefits will be more or less the same. Dust mite-proof covers are effective in that they are impermeable to dust mites and easy to remove and wash. So, when dust mites settle and lay their eggs on your bed, they will be on your washable blankets, sheets, and dust mite bed covers.
Use a commercial dust mite killer. The steps listed above are the most effective methods for getting rid of dust mites. If, however, you want to speed the process up a bit and kill dust mites with dust mite killers, consider using Acarosan (active ingredient Benzyl benzoate), Steri-Fab (active ingredients synthetic pyrethroids, isopropyl alcohol, and a dual quat ), or Dust Mite & Flea Control by The Ecology Works (active ingredient borate).
For those of us unlucky enough to have a dust allergy, dust mites are the culprit. Found in dust mite carcasses and dust mite feces is a protein called DER p1. Most people's immune systems couldn't give a crap less about this protein. Unfortunately, the immune systems of people with a house dust mite allergy are kinda stupid. They mistake DER p1 as hostile and go on the attack. This gives rise to such dust mite allergy symptoms as drippy, runny noses; stuffy noses; coughing; sneezing; teary eyes; itchy eyes; itchy nose; itchy throat; itchy roof of mouth; chest congestion; wheezing; skin rash; post-nasal drip; chest congestion; swollen eyes; red-rimmed eyes; dark circles under the eyes; crusty eyes; and facial pressure and pain, just to name a few.
Reactions to the dust mite allergen can be even harder on those with asthma. Airways in the lungs can become angry, swollen, and inflamed, increasing the severity and frequency of asthma symptoms. And just in case you don't hate dust mites enough already, they have also been linked to problems like rhinitis, eczema, atopic dermatitis, and, albeit loosely, SIDS.

Apply pressure to other house pests. Dust mites are fond of eating the poop and the sheddings of other pests like fleas, roaches, and silverfish. By removing these things from the dust mites' diet, controlling dust mites will be easier. Check the homepage of this site for pest control information on those and other unwelcome guests.

Tannic acid dust mite spray. Products containing tannic acid, such as Allersearch ADS Anti-Allergen Dust Removal Spray, do not actually kill dust mites. They do, however, denature the protein that causes dust mite allergies.

Eucalyptus. This plant is a natural dust mite repellent. It has been suggested that you can reduce house mites by scattering eucalyptus cuttings around the outside of your home and by decorating with them inside. You can also add a couple dozen drops of eucalyptus oil to your wash for killing dust mites in laundry.

Pets. Dust mites love pet dander too. Make sure to wash pet beds as frequently as you do your own. You should also give your pets treats with, or supplements containing, omega fatty acids. This will cut down on pet dander by helping to keep their skin healthy.