White tongue common. The tongue is known to have many variations with pigmenting, size, appearance, & not always due to disease. Causes of a tongue that looks white include: lichen planus (responds to steroids), white sponge nevus (familial, benign, permanent), smoker's leukoplakia (2% risk of cancer, needs biopsy), hairy leukoplakia (due to epsteinbarr virus), warts (due to papillomavirus), "thrush" (rx antifungals).
Thrush. A common cause of a white coating on the tongue is thrush, a fungal infection most commonly the result of a disruption of the normal bacterial flora of the mouth. See your family doctor for an evaluation.