Duranium was a naturally occuring, extremely strong metallic, substance. Due to its abundance in numerous planetary cores, duranium was used for hull construction for starships and facilities by numerous races across the Milky Way galaxy. (DS9 reference: Star Trek: Deep Space Nine Technical Manual)
In combat situations, duranium shadows can be used to fool enemy sensors into believing that there is a large stockpile of weapons located at the position that they are scanning. In 2369, Chief Miles O'Brien and the crew of Deep Space 9 were able to fool Gul Jasad and his attack force that the station was more heavily fortified than they believed. (DS9 novelization: Emissary)
In 2377, a survey team from the USS Ottawa identified a major duranium deposit on the planet Grecon IV in Cardassian territory. The Ottawa crew agreed that the deposit would be of significant importance to the Cardassian economy. (USS Baldwin: "Like the Rain")
Duranium was a substance much stronger than either Transparisteel or Bronzium. It was extremely durable, light, and was capable of withstanding a glancing blow from a lightsaber; however, it also had a very high melting point, making it difficult to shape. It was however, fool proof against blasters and slugthrowers, only the continuous heat of a lightsaber able to melt through it.
"Duranium" is also a strong substance in Star Trek, used in the construction of starship hulls in that universe; it was first used in the Original Series episode "The Menagerie," predating Star Wars itself by nearly eleven years.