It is a type of crown/headband worn to represent royalty.
Ravenclaw's Diadem is a silver crown with the blue jewels in the middle. See related links for a picture of the film prop that was displayed at Comic Con 2010.
Vincent Crabbe destroyed the diadem when he used fiendfyre (cursed fire) on it, while trying to kill Harry Potter (along with Ron Weasley and Hermione Granger).
Well, actually Rowena Ravenclaw's magical Diadem was stolen by her own daughter - The Grey Lady - the ghost of Ravenclaw House. After that The Grey Lady flees. Rowena asks The Grey Lady's lover to go & find her but instead he kills her. But before that The Grey lady manages to hide The Diadem in a stump of a tree by magic. The Grey Lady's lover was The Bloody Baron - the ghost of Slytherin House. He finds and kills her, I don't remember why. That's the reason he has blood on his robes. Many years later Tom Riddle discovers from The Grey Lady the location of the Diadem, finds it and turns it into a Horcrux.
Yes he found it at the end of the last book, it was in the ravenclaw common room.
the diary the, the ring, ravenclaw's lost diadem, the snake Nagini, the locket, hufflepuff's cup and Harry, himself
She adorned her head with a sparkling diadem for the royal ball.
diadem
diadem seems to fit.
Diadems are:1. A crown worn as a sign of royalty.2. Royal power or dignity.di·a·demed, di·a·dem·ing, di·a·demsTo adorn with or as if with a diadem
I am wearing a diadem.
The diadem was one of Tom Riddle's Horcruxes. A diadem is like a sort of crown.
Diadem of gold or precious stones.The queen wore diadem of jewels and gold
Operation Diadem happened in 1944.
A zodiac diadem is an ASTROLOGICAL CROWN. or something very similar to that.
Madonna with the Blue Diadem was created in 1511.
Mograbin diadem snake was created in 1923.
Many Greek goddesses actually wear a diadem in Greek mythology, as it is an emblem for regal power or dignity. Back in Greek society however, the diadem also suggested a superhuman status, and demanded respect that was shown to the Gods. Given that Hera is the wife of Zeus, one can speculate that she wore a diadem to show that she is the Queen of the Gods, and therefore demands respect for her position. An interesting thing to note, however, is that many Gods connected with death and the afterlife were seen wearing diadems - feel free to find a connection between this fact and Hera.