There were 7 shenlongs, each captured in the essence of a dragonball, the black star balls are the universe balls, nothing special. They use these balls to bring back the earth after Baby destroys earth (indirect affect) by wishing for Planet Vegeta to come back.
the 7 shenlongs were a recurrence of the negative wishes on the dragon balls of earth, the unltra-soft underwear, Slug' s eternal youth, etc.
Goku and pan beat all 6 of the shenlongs, each shenlong represented something, like water or earth or electricity, and each opposite defeated the shenlongs accordingly. Goku was not strong enough to kill the last shenlong, as he absorbed all 6 balls and became very strong. Goku and Vegeta were the same pl after going ssj4, they maxed out at a billion. They fused to form gogeta, a pl of 1.5 bil. They still couldnt beat him. eventually they got one of the absorbed balls and was able to beat him, goku was still a kid and couldnt refuse after they unfused cuz of the size difference. Then Gt ends with son Goku and Son Vegeta going ssj (the last of the race that can go ssj) and pan sees goku in the crowd, but when she gets down to where he was, hes gone... Gt ends, and thus the end of the drgaon ball series, for now at least. It might come back if its popular in the States. Thats what the maker of Dbz said, and he wasn't lying.