What does "update my facebook status" mean? How do you do that? And why?

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I don't have a facebook and when I've browsed accounts I never see a "facebook status". I was wondering what that is and why people do that? Also do you have a drop down box where you pick & choose or can you just type in any nonsense?

And what does the status do in relation to facebook/others etc? I only seen myspace (sux) & Twitter and they don't have no status to update.

Also I heard that when Michael Jackson died people had RIP MJ as status. I can't believe that was preprogrammed in a drop box so how does it work & why is it necessary.

Sorry for my ignorance but if you tell me then I will know, thanks! =)

Thanks.
Thanks John. Much appreciated. I'll give you BA as soon as I can pick it!