Dungeons:
Dungeons are basically places in random parts of the World Map that contain Different Challenges, some harder than other but if your team of (5) is communicating through either text or Voice chat and everyone is playing there best then you should have no problems completing the many Dungeons. The Higher you get the more Dungeons you get to have available. Although you need to be a lvl 15 to start the lowest Dungeons, it really help you gain Exp to rank up your lvl and you now get Justice Points that you can spend on items at a Justice Quartermaster Vendor in Some major cities. For Example: An Alliance "JP" vendor is in The City Stormwind and its in The Old Town area in the back. For another Example: The Horde there is one in Ogrimmar in the middle of The Valley of Strength.