Candidiasis

More commonly known as oral thrush, candidiasis is a yeast infection often seen in infants and the elderly as well as those who wear dentures and have weakened immune systems. Candidiasis can also occur when taking antibiotics and can easily be remedied by eating plain yogurt to restore the good bacteria.

Leukoplakia

Leukoplakia is an excessive growth of cells on the tongue that will form a white patchy coating. While not inherently dangerous, it could be a precursor for cancer, but only a dentist can tell for sure. Leukoplakia is often found in people who use tobacco products.

Hairy Tongue

Hairy tongue is an over production of keratin that results in a white "hairy" coating on the tongue. Keratin cells sit on the top of the tongue and help protect the it from rough foods. A soft diet can cause a build up of Keratin and can be easily removed with a tongue scraping or using oxygenated toothpaste.

Dehydration

When the body does not have enough liquid intake compared to what it is losing, the body cannot carry out normal functions and as a result, a thin white coating can appear on the tongue. The easiest way to prevent this from occurring is to keep the body hydrated.

Tonsil Stones

While not common, tonsil stones can be caused by an excess build up of anaerobic bacteria that can form on the back of the tongue, leading to a white coating as well as hard deposits forming on the tonsils. A physician can tell if the white coating is the result of tonsil stones.

Source:

MedicineNet

Mayo Clinic

More Information:

American Academy of Oral and Maxillofacial Pathology