As a general rule, synesthesia can occur between just about any combination of senses or cognitive pathways.
The most common type of synesthesia is grapheme-color. Graphemes (letters or numbers) take on certain colors to the synesthete. This is almost always uniform for each person, but not among synesthetes in general. For instance, one person with synesthesia could always see the letter A as red, but that doesn't mean all synesthetes will experience the same color. (Although there are some studies that show certain combinations appearing more among synesthetes -- including A as inherently red [source: Ramachandran and Hubbard].)
Another synesthesia that is reported is sound-to-color synesthesia. This could indicate a few different things; it might mean that something as simple as a noise (a car horn, flushing toilet) takes on a color or shape in the mind's eye when it's heard. You might experience some sounds as a texture. Some lucky folks get to "see" music and associate a wide variety of musical notes with a rainbow of colors. Interestingly, a 2006 study did show that sound-to-color synesthetes associated a higher pitch to a lighter color -- which non-synesthetes also agreed on [source: Ward]. This led researchers to conclude that synesthesia might be using the same cross-modal pathways that most people use, rather than pathways special to the condition.
Next up is one of the rarer (and more delicious) forms of synesthesia, which is lexical-gustatory (or olfactory). This kind of synesthete might taste certain flavors (or even whole dishes or meals) based on a visual picture or word or sound. They might also associate smells with certain colors, or shapes -- let's say the smell of toast might be ochre. Some synesthetes even claim to taste the emotions of other people [source: Carlsen].
Some less common forms include mirror-touch synesthesia, which causes some people to experience a physical feeling when other people are touched (read Can people feel the pain of others? for more info on this particular twist). Ordinal-linguistic personification is where a person might give personality traits to individual items in ordinal lists. For instance, saying that the letter A appears egotistical, the letter B nurturing and the letter C brave -- it can happen with numbers, days of the week and so on. Number-form is where a person sees numbers as a very distinct map -- often, lower numbers (more commonly seen ones) are bunched up at one end (or the first 12 might imitate a clock face). They're not necessarily linear, but might curve in and out, or move up and down. However the map "looks," it stays uniform for the number-form synesthete.
So we got it: There are lots of synesthesia forms out there. But why in the world do they occur?