This is one of the most bizarre and interesting phenomena's of human perception. A lot of people don't even know that they have it because they assume that everyone else perceives the world in the same way that they do. Its basically when your senses get jumbled and linked in strange ways with each other. There are some people who "taste" words, and others who see numbers in color. Its ironic that I saw a documentary about this once about a guy who "tasted words" and he specifically said "Derek" tasted really bad! (not true!)
Some people are born with it while others develop it due to brain injury. The most fascinating aspect of it is that in some cases it imbues some persons with an almost added ability that someone without it might not have.
"Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae or synaesthesiae), from the ancient Greek σύν (syn), "together," and αἴσθησις (aisthēsis), "sensation," is a neurologically based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes.
In one common form of synesthesia, known as grapheme → color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored, while in ordinal linguistic personification, numbers, days of the week and months of the year evoke personalities. In spatial-sequence, or number form synesthesia, numbers, months of the year, and/or days of the week elicit precise locations in space (for example, 1980 may be "farther away" than 1990), or may have a (three-dimensional) view of a year as a map (clockwise or counterclockwise). Yet another recently identified type, visual motion → sound synesthesia, involves hearing sounds in response to visual motion and flicker. Over 60 types of synesthesia have been reported,but only a fraction have been evaluated by scientific research.Even within one type, synesthetic perceptions vary in intensity and people vary in awareness of their synesthetic perceptions."