Its far easier to look it up than have me explain it. Its extremely complicated, and if i tried explaining it, i would for sure mix things up. Wikipedia has a fairly detailed explanation of it though, if anyone is willing to read through it.
Basically, People see colored words/letters ( whole and individually) when they either hear the words spoken, or read them.
Some people Taste things when certain words are spoken, or read.
Its quite interesting.
http://en.wikipedia.org/wiki/Synesthesia