I've always thought of the hook of a song as being anything that grabs the listener's attention and pulls the song together...It can be a line, a chorus, or even a really catchy riff that sticks in people's minds and helps define the number as its own entity. I find the phenomenon is more noticeable in pop/rock music, where the entire song is based around this type of attraction, but its appeal covers all genres... I think I often get the turnaround mixed up or blended with the bridge...I'm a little confused there myself!