It manifests in many different ways, but the crux is this: it’s an overlap of two (or more) senses, such as sight and hearing, or hearing and taste.

For instance, some people see colors. Other people smell or taste words.

The woman on TV had a color that was associated with each letter of the alphabet (H is green, R is magenta, etc.). Oh, and everything she hears is displayed to her on a ticker tape, one letter at a time, in front of her face.

It’s hereditary. Her brother has the same thing. But they don’t necessarily associate the same color with the same letter.

The other guy had a smell associated with many of the words that he heard. “White” could be marmalade and “Hello” could be breakfast sausage.

For many years the scientific community thought these people were just making the stuff up, or that they were using drugs. However, recently the condition has been the subject of several scientific studies which have confirmed that not only is the condition real, but that it affects about 1 in 300 people.

Many of the people with this condition have always had it, so they have never know life any other way. Several of the people on the show were surprised to find out that not everyone perceived the world the way that they do.

Source: The Science Channel

There’s a wiki page but it’s not even close to as interesting as the TV show.