There are currently eight distinct leagues which you can play in poe currency, each with its own market and Path of Exile players. Because it's been around for so long, all of the items that you could ever want have been crafted, and therefore are quite cheap because of demand and supply. Builds in Standard are, generally speaking, stronger and cheaper.
Leagues, on the other hand, are temporary, and most Path of Exile players spend the majority of their time playing in a league. Each league comes with its own gimmick. The present league is 'Harbinger,' and they have added packs of Harbingers that spawn randomly points in channels, in addition to new currency and an extra map.The launch of War for the Atlas, the most recent expansion to Path of Exile, brought with it lots of new skills and an equally high number of modifications to existing builds.
The Witch in PoE is much like caster classes in different RPGs. Arc, as its name implies, is a lightning-based ability that features the capability to string from one enemy to another. The number of enemies that Arc can chain to will be dependent upon a particular character's gem and enchantment configuration, but in any event it can't chain over 11 times per cast.
As you can imagine, when every throw of your skill is bouncing around and hitting around 11 distinct enemies, then the DPS can add up fairly quickly. Bear in mind that the DPS mirrored from the tooltip isn't telling you the entire story (as is frequently the case with most tooltip DPS, on most abilities, in the majority of Path of Exile games). The tooltip DPS is for each individual enemy hit, so you would need to multiply the tooltip amount by yet many enemies you are hitting at once with the capability to be able to get a more accurate read on your DPS.