The Parni peoples originated form thePanj River valley, but rose to prominence in 247 BC, when a man named Arsaces I was elected Chief of the Parni. At the time, the Seleucids were experiencing large-scale turmoil, and the Satrapy of Parthia seceded the Empire, in favor of independence.
The Parni capitalized on this, and invaded all of Parthia, from whence, they expanded northward, since their southern border was next to the mighty Seleucids.
In 211-209 BC, the Bactricians and Parthians went to war to decide which nation would be predominant over the other in Central Asia. That war ended in a stalemate, but the Parthians launched a surprise attack in 197 BC, when they took Samarkand, winning the war for the trade routes.
The culture of the Parthians is a mixture between Persian and Hellenistic influences. Since the Seleucid Empire is so near to the Parthian homeland, the Hellenistic culture widely defines what it means to be a Parthian. Greek theatre is commonplace, but the Arsacid dynastic rulers tend to try to associate themselves more with the Achmeniand Dynasty than that of Seleucus.
The main religion, and only practiced religion with a large following, in Parthia is Zoroastrianism. The religion of the Achmeniand Empire, Zoroastrianism's polytheistic form allowed for the Greek influence to continue, since many equated the gods of Zoroaster to Zeus, Hades, Hermes, and other Greek gods.