*CUT TO THE CHASE*
EVERYONE DIES AND THE MILITARY NUKES MANHATTAN.
They die in this order:
1) Jason Hawk (Rob's brother) - dies on the Brooklyn Bridge when the monster attacks while everyone is trying to flee Manhattan, this happens about 20 minutes in.
2) Marlena - She gets bitten by one of the mini monsters in the tunnels and ends up exploding once they reach military help in the mall
3) HUD - Gets mauled by the monster right near the very end after the helicopter crash while getting PERFECT footage of the monster.
4+5) Rob and Beth - Supposedly get blown to pieces after the military imposes the HammerDown code and nukes the city. After identifying themselves on tape and embracing, telling each other they love each other.
Quick note:
Lily gets into a helicopter and is separated from HUD, Beth and Rob. It is not really known what happens to her. They don't show her helicopter wrecked.
Also, if you wait after the credits you hear radio static and something that sounds like a distant roar.
THERE IS NO BACKSTORY INTO HOW THE MONSTER COMES TO BE! BUT IT DOES LOOK EXACTLY LIKE THIS:
pic 1
and
pic 2
The little monsters look like the Uglies from Starship Troopers. The film doesn't really go into them either, except for the fact that they fall off the big monster a lot like barnacles. And they sound like someone nashing there teeth loudly. NYAH NYAH NYAH type sound.