Here's a very pretty version of a condition shared by two to three percent of the population. It causes patterns, often map-like patterns, on the tongue.
Normally, the human tongue is covered with tiny papillae. These hairlike structures can look pink or white, and they look a bit like a film over the top of the tongue. But people with geographic tongue have bare patches on their tongue. These patches show darker pink or red against the rest of the tongue, and because they lack papillae they have a different texture.
The tongue above has exotic-looking curlicues, but most people with geographic tongue just have patches that look like pink islands in a white sea. They make the tongue look like a map, which is how the condition gets its name. The patches fill with papillae over time, but other spots lose their papillae, so the islands usually migrate over to other parts of the tongue. The lingual geography shifts.
Although geographic tongue may be more common in people with psoriasis, it's not dangerous. The largest health effects are a slight burning on the bare patches when eating spicy food. The most severe complication listed by the Mayo Clinic is "It may be difficult to be reassured that there is, in fact, nothing wrong." Geographic tongue is just another interesting variation in the human body.
[Sources: Geographic Tongue]