"A hook" can be anything that catches the listener by surprise the first couple of listens, awakens one's interest and/or attention, or something that makes one mess his/her pants. It could be an odd note here, a nifty drum fill there, a sudden change, etc.
The most common hook (which has become more of a cliché) in pop, dance and techno music is playing the chorus, then transposing it up half or full a step and playing it again.