(Some of the names/terms here are LIT (lost in translation) so they might differ from other sites, also the whole story of this is told in so many different ways, this is just what I have learned from the manga, so there is most likely a few facts left out.)
Millennium and Sennen mean the same thing, one is just for the Japanese and one is for the English. They are called that because Atem's father believed that with them they would go into a new millennium. They were originally created by Yami�s father Akhenamkhanen and his brother. They sacrificed 99 lives in the city of Kul Elna to make the items and they then chose �holders.� The millennium items were most often used to �try� souls, but they also brought along with them darkness. Bakura came from Kul Elna and tried to possess all of the items so that he could bring them back to his city where they were created. The city is now in ruin, but a shrine is hidden there, it is said that the person who places all seven items there will gain the power of the shadows- of Zork Necrophades, who is an evil God.
The millennium Puzzle ~ This one is obviously held by Yuugi/Yami, it is held by the pharaohs of Egypt. Most of its powers are hidden, but you are able to being out the shadow realm with it. Yuugi�s grandfather, who was an archeologist at the time, found the puzzle in 1960 in the Valley of the Kings. Yuugi took more then 8 years to solve the puzzle after he received it from his grandfather. Inscribed on the box that it came from was, �The one who solves me shall gain the powers and knowledge of the shadows.� It was also believed that it would grant you a wish, to which Yuugi wished that he could have friends. The ancient pharaoh Atem was locked in the puzzle, and was able to possess Yuugi after he solved it.
The millennium Ring ~Originally owned by the priest Mahado, as well as Thief King Bakura, and is now owned in the present by Ryou/Bakura. It has the power to detect the other millennium items, but also to detect �evil� Ka. That is why the priest Mahado was in charge of keeping the tombs safe because he SHOULD have been able to detect the bad Ka. However, as times went on there were more and more sinners so it made it much harder. The present Bakura simply uses the ring to find the other millennium items so that he can steal them.
The millennium Rod ~In the past was held by the high priest Set, who is later reincarnated as Seto Kaiba, but later is held my Malik/Marik Ishtar.(^^) You are able to control minds with the rod, but also in ancient Egypt was used to seal the Ka (the sinners �soul� monster) inside stone tablets.
The millennium Eye ~Owned in the present by both Bakura, and Pegasus. In the past it was owned by the high priest Akhenaden, who used it to reveal the true form of some one�s Ka. Pegasus used it more or less to �peak� into their mind to find out their next move. Yugi however, won the match and in the end left Pegasus weak. Bakura took advantage of that and overpowered him and took the eye for himself. Later on in the series he gives it to Yami no Yuugi.
The millennium Key/Ankh ~ Owned entirely by Shadi (though technically in the past he was the high priest Shada), though he more or less gives it to Yami no Yuugi. It is held onto by Bobasa. The millennium key allows the holder to look into a person�s heart, showing whether they are good or evil. Also one is able to look at some one�s soul.
The millennium Scale ~ Held by Shadi, so therefore past to Yuugi, and held by Bobasa. High priest Karim held it in the past. They use it to �weigh� ones soul. Based on the Egyptian belief that when one dies they go through a trial where they try ones soul. One side holds the feather of Ma�at, (who was the patron of truth, law, and universal order.) She would be in the feather and try the person�s soul, with the feather on one side the scale was even. When they place the heart of the (usually) deceased person on the scale and it remained evil the person was known to be a honorable person in life. But if the scale tipped they were fed to the Ammit which would eat the soul. So it pretty much weighs evil.
The millennium Necklace/Tauk~ In the present is held my Malik�s elder sister Isis/Ishizu, whom is the reincarnation of it�s previous owner the High priest Isis. It pretty much allows the holder to look into someone�s future. Although a lot of the time it is really cryptic.
- Meh, there are obviously pictures of the items around, but until we get our barrings on the site we wont put any up ^^ But there will be, don't worry.