Synaesthesia was known for more than 300 years and has influenced much famous artwork and poetry, but it wasn't studied extensively till recently.
Synaesthesia litterally means the joining of senses. It is a condition in which someone will get a stimulus in one sense, which will give a sensation in another. Common types of synaesthesia include seeing sounds, seeing letters that are actually written in black-white in colour or tasting shapes. I myself see the Braille I'm reading in colour. (I still have some sight and know what colours are.) I'll also "see" colours in things I'm touching, even without looking at them. Once I thought I "saw" something was light-grey, while when I looked at it, it appeared to be black.
As stated above, there are many types of synaesthesia. If you mix each sense modality with another, there are about 72 synaesthesiae. However, some appear not to occur, such as taste-smell synaesthesia. Also, it's hard to determine if a person who tastes smells or smells tastes is really synaesthetic, cause the senses of smell and taste are closely related. I myself for example can taste smells, but think that's mainly because strong smells could make one "taste" them cause the senses of taste and smell are connected.
There are various theories of what causes synaesthesia, including the idea that it is a more primitive way of perceiving stimuli and related to disorders like Schizophrenia, the theory that one compensates for a defect in handling stimuli on going to a "superior auxillery sense", and the idea that colours and sounds are simply related mathematically. It is also thought that people associate colours they like to things they like. Synaesthesia is also thought to be common or even standard in infants up to age three months, meaning they at least can see sounds but might have other synaesthetic perceptions. It is also thought that some events later in life - such as brain damage or blindness - might trigger synaesthesia.
Is synaesthesia a disorder? To me it's not, nor is it a disability. It's a neurological difference, but rather a gift than a problem. However, in combination with other disorders, itmight be very confusing and being perceived as a disability.
