The Goblet of Fire is the "impartial judge" for the Triwizard Tournament that chooses which students will represent their respective schools. After the students have finished placing their name in Goblet, the Goblet then chooses one student from each school by spitting out their name in a rush of red flames. Once chosen, the participants are subject to a binding magical contract in which they can not back out of the tournament.
It is not a collectable object.
From the Story
A large, roughly hewn wooden cup, the Goblet of Fire is the impartial selector who chooses the three Triwizard champions. It is kept in a large, jewel-encrusted wooden chest when not in use.
Discovered in Harry Potter and the Goblet of Fire
The Goblet of Fire is brought into the Great Hall in its chest by Argus Filch, who places it on the table in front of Professor Dumbledore. When Dumbledore takes the Goblet of Fire out of the chest, it is filled to the brim with dancing blue-white flames.
Anyone who wishes to try out for the position of Triwizard champion must write their name and school clearly upon a slip of parchment and drop it into the Goblet of Fire. Prospective champions have twenty-four hours from the time the Goblet is activated to place their names in the Goblet; it will return the names of the three students it judges most worthy to represent their schools at the Hallowe'en feast at Hogwarts.
As each name is dropped into the Goblet of Fire, the blue-white flames briefly turn red and emit sparks. When the Goblet is ready to make its decision about the Triwizard champions, it shines more brightly than anything in the Great Hall, the sparkling bright, bluey-whiteness of the flames almost painful to the eyes. The flames suddenly turn red again, and sparks begin to fly from it, before a tongue of flame shoots into the air accompanied by a charred piece of parchment - the name of the first Triwizard champion. The flames turn blue-white once again, and the Goblet repeats the process twice more to reveal the three Triwizard champions. Just as Dumbledore starts his closing speech the fire in the Goblet turns red once again. Sparks fly out of it and another piece of parchment is ejected - the name of a fourth Triwizard champion, Harry Potter.