The Gresious Orb is a special item introduced in Pokemon Platinum version. It is designed specifically for Giratina to hold, but can still be held by others.
The Gresious Orb will boost the power of Ghost and Dragon type moves for the user when being held. If held by Giratina, though, another effect will take place. Giratina will transform into his Original Form, and will stay this way, even in the normal world. But, if you take it from him, he will transform back into his Alternate Form. This changing effect will take place instantly, featuring a small animation in your party screen, whereblack smoke will surround Giratina's sprite, he will let out his cry, and the smoke will clear, revealing his transformation. A message will appear, saying "Giratina transformed!!".
Most people think that the Original Form is more powerful than the Alternate Form, so having the Orb is very useful.
To obtain the Gresious Orb, you will need to beat Cynthia, enter the Sinnoh Hall of Fame and finish the game. Then, go to Turnback Cave and get to the Portal Room. Enter the portal to the Distortion World, and make your way to the end of the small path. The Gresious Orb will be waiting for you there.