The seven rings were the Rings of Power given to the Dwarves by Sauron to seduce them to his service. However, the only real effect the rings had on the bearers was to make them lust even more after the riches of Gold and other valuable materials.

They had metal bands and were each set with a single stone. The greatest of the seven was the ring of Durin’s line - the Ring of Thrór.

Due to his failure in corrupting the Dwarves, Sauron tried to recover the Seven. He was successful in obtaining only three, the others having been consumed by Dragons.
Encyclopedia entry originally written by Mirlomien