Full Dragon is the one most legendary and prestigous armour set in Runescape. The term full is player jargon and refers to a complete melee armor set consisting of four parts: full helm, platebody, kiteshield and platelegs. The melee armour sets from low to high are as follows: bronze, iron, steel, black, mithril, adamant, rune & dragon.
There is also the Barrows armour, the Granite armour and the Dragonhide armour for ranger and various robes for mages but neither of them really fit the full armour concept of bronze, iron, steel, black, mithril, adamant, rune and dragon. Unlike the other six armour types, Black and Dragon cannot be smithed from metal bars and they must be found. Black can only be found at monster drops and Dragon can only be aquired through Quests or certain monster drops.
Note that the full armour standard does not come with just any helm, body, legs and shield - it refers to the full helm, platebody, platelegs and the kiteshield. The chainbody, medium helm, chainskirt and the square shield are not parts of the full armour standard. But in the case of full dragon there was initially no platebody or full helm and there still is no kiteshield available in the game and therefore the full dragon came to consist of Dragon med helm, Dragon chainbody, Dragon platelegs and Dragon square shield. The Dragon full helm was released in 2007, the Dragon platebody in 2008 and the full Dragon is now closer to what it should be: Dragon full helm, Dragon platebody, Dragon platelegs and the Dragon square shield.