Haha, it could be the next step in evolution although most of what I read says its an under-developed brain. Its commonly associated with autism.
Basicly it's a cross-wiring of the brain, studies show that when I hear something, my visual cortex lights up on a brain scan. Its fun
I'm not sure how to describe it. The easiest one to describe is that letters and numbers have colors. For example T's are always green, M's are always red.
Your name to me is
M- red
A-blue
N-orange (except for some reason lowercase is brown)
D- Yellow
I- Mustard yellow
E- red
P-bright Yellow
oo- these go together like a periwinkle blue- very vague

That's what I see if I read your name and pay attention. I can tune my synesthesia out
when reading because text is black and white but if I think about what I read it's in color.
Days of the week and months and times and seasons are colored separately, haha. I don't like Saturdays because the are grayish all day. Also November is chronically brown.

But my personal favorite form I have is color-sound. When I listen to music I essentially experience my own personal light show. The colors come and go with varying sounds. Different intruments have their own color and shape, different notes on the same instrument have different shades, and or colors, and certain scales of music have different colors. I used to experience all these colors at once but since I have been on psych meds I only experience the ones I "tune into" or choose to focus on. Its much less on psych meds (I take anti-seizure meds Lamictal/Trileptal)
The only downside is I find myself listening to a lot of music that a lot of people find to be trashy like Britney Spears - Three is this awesome vibrant red, pink, orange, candle-flame type ribbons of color that are super fun!
Classical music is tough to hear because there is just so much going on to process that I'm exhausted afterwards. I've been known to go into sensory overload easily if there is too much noise, "Tv, radio, people talking all at once"
What's interesting is that these colors haven't changed since birth and I dont think they will. I thought it may have been from watching too much fantasia as a kid but it's not.

Its good until its bad. Sometimes the smell of my boyfriend if he hasn't showered isn't regular B.O. its these sharp orange needles in the back of my throat. Vanilla is always a ribbon that wraps around my like a fuzzy blanket. Kind of like the febreze commercials.

These colors don't obstruct my actual vision, they kind of hang out in the front of my mind. Because I'm hearing them so they arent processed along with whats actually in front of me.

I started having these weird hallucinations back in february after starting risperdal. But they weren't real because I knew the objects I saw were fake and my reality testing remained intact. At the hospital they couldn't classify me as anything but the attending had a theory that because of synesthesia these things I was experiencing was not a far stretch of the mind. Clearly some wires were obviously crossed combined with bipolar mania equalled madness!

Sorry this was really long but I just get so excited about it.