What is the "hook" of a song?

[Brethren]

I've been wondering over this, and I've never grasped the meaning of a "hook" of a song.

Can someone please take the liberty of explaining it to me? Thank you so much. :)

[Unexpected]

The hook is simply the part of the song that "grabs" you; the part that comes off most memorable. In songs this is usually the chorus.

Call-out hooks are used to determine if a song tests well or not.

But I think the song has to be the total package for it to be a good song, not just the chorus, usually.

[Unexpected]

Actually if you wanna get technical there are 5 types of hooks:

Melodic - sections of songs that make you wanna hum the melody.

Lyrical - part of the song that summarizes the message and creates curiosity and identifies with people. Usually the title of the song.

Musical - musical hooks can be riffs, licks, or figures. e.g Guitar sample from "Ventura Highway" for Janet's "Someone to Call My Lover." 
 
Rhythmic - pretty much anything done in a rhythmic matter like the "hah hah hah" parts in the Bee Gees' "Stayin`Alive"

Sound-effect hook - exactly what it says like in Brooke Allison's minor hit "The Kiss Off (Goodbye)" When they use the famous AOL "goodbye" sound byte at the end of the chorus.

"Turn of the Light" by Nelly Furtdao and "Music" by Madonna both use many of these different style of hooks.

[jimmy74747]

The melodic & musical hook referred to above are probably the two that people most often refer to.

[mst3k]

The hook brings you back. On that, you can rely.

[strong4PMB!]

If you're having problems with a song, I'll write a hook for you. - Christina Milian, "A Girl Like Me"

[HER MINAJesty]

When I say "hook" I mean the 'chorus' of the song