Leukoedema refers to a benign discolouration on the insides of a person's cheeks.
Leukoedema is actually pretty common. It is more common in people with darker skin, and it can also be caused or intensified by tobacco use. As many as 90% of adult blacks and 50% of adult whites show at least minor discoloration, making it not a disorder but actually the norm. Severe discoloration is less common, but still harmless.
Leukoedema may be grey, white, or even bluish. The same discoloration might appear on any mucus membrane, but technically it is not leukoedema unless it appears on the inside of your cheek (your buccal mucosa). Sometimes discoloration of larynx, soft palate and oral floor will also be (technically incorrectly) called leukoedema. It is typically bilateral (present on both cheeks), asymptomatic, and permanent. It generally appears in late childhood or the early teens, but generally no-one notices. It may be accompanied by an odd, lacelike pattern of grooves in the mucosa.
Leukoedema is characterized by edema in the outer epithelium of the buccal mucosa, and often is accompanied by damaged or dying cells. Parakeratosis ("callusing") is common, but not as pronounced as it would be if resulting from trauma. No treatment is warranted, and as far as I can find there is no known treatment.
Don't stop worrying yet! Remember how I said that leukoedema was a benign discoloration? There are other discolorations that are not benign. Sudden changes in the color of the inside of your cheeks, bleeding, odd textures, and odd feelings might be signs of something worse than simple discoloration. If you are in doubt, check with your doctor. If you are seriously going to base your diagnoses on something you read on the internet, you can check your own leukoedema by stretching your checks out -- leukoedema will typically disappear when the tissue is taunt. If it does not, you really should go see a doctor.
If you aren't too squeamish about looking in people's mouths, here's an extreme example.
Not to be confused with leukoderma, meaning discolored skin.