Ear mites look like tiny little spiders under the microscope. Basically they are a parasite.
If your dog has an itchy ear with discharge in it, you should take them to the vet. The vet will take a swab of the material and examine it under the microscope. Common causes of ear infections are ear mites, yeast, or bacteria. Depending on what the vet sees under the microscope and after looking down the ear (to be sure the ear drum is intact and see how much inflammation there is in the ear canal), they will prescibe an ear cleaner and an ear medication. Usually, the vet will want to recheck the ear in about 2 weeks to ensure the infection is completely cleared up.
Mites are not all that common in adult dogs as they are in puppies and kittens.
Weekly ear cleanings are a good idea to prevent all types of infections in the ear. Check with your vet about what ear cleaner they recommend. I use OtiCalm for maintenance in my dogs.