Path of Exile, unlike other games of this genre, has a different type of currency system - there is no "gold" in the game, we use a variety of items called "orbs" and "scrolls". These are used for trading with other players, as well as crafting or improving items, identifying magics, rares and uniques, or even to open a Portal to Town.  Here we will talk about main currency types, what they do and which items they can be used on, without full detail but giving examples to clear it up. 

There are two types of currency in Path of Exile - basic currency and league-specific currency. 

Basic currency drops from monsters and chests in both Standard and Challenge Leagues, Softcore, Hardcore and even in Solo-Selffound Leagues, so basically everywhere in the game. 

League-specific currency only drops in the related Challenge League, or - if they are implemented in the Standard League - from their related League specific bosses, zones or maps. We can exchange some of the currency with vendors in Town to items or other currencies, but we usually get a better value by trading with other players. 

There are certain item sets which can be exchanged with vendors for a valuable currency. These so-called "vendor recipes" can reward us for example with Chaos Orb(s), Orbs of Alchemy, Jeweller's Orbs, or Orb of Fusings. If we sell a full rare item set (means it is capable of filling all item slots in the inventory, exluding Flasks, alternate weapons set or a Quiver) above a certain item level to a vendor in town, we can get a Chaos Orb, if they are all unidentified, we can even get two or three Chaos Orbs. Or if we sell a Six-Socket item to a vendor, we can get seven Jeweller's Orbs. There are many other of these vendor recipes, some are related to maps or items, or even to Skill and Support Gems.

Many of the currencies can be used to craft powerful rare items, changing their attributes or values on them, while others are used to increase the number of sockets on an item, or link these sockets. With Orb of Fusings, we can randomize the number of linked sockets on an item to create a Six-Linked item so we can support our favorite Skill Gem with five different Support Gems to make it much more powerful. A Regal Orb is capable of upgrading a Magic item to a Rare, keeping it's stats. With a Chaos Orb we are able to reforge all the stats on a rare item to get completely new ones but a Divine Orb is able to randomize only the numeric values on the item, keeping the modifiers. With an Orb of Scouring we can remove all the modifiers from an item, making it a white base item. While for example a Chromatic Orb is able to recolor the existing sockets on an armour or weapon.

Some of the currency can only be used on armours, weapons, maps or Flasks, while some can be used on all. A Glassblower's Bauble can increase the Quality on a Flask of any type, but it cannot be used on any other item. A Cartographer's Chisel can be used to increase the Quality of a Map, but it also cannot be used on anything else aside from maps. Armourer's Scraps and Blacksmith's Whetstones can increase the Quality of Armours and Weapons respectively, but they cannot be used on any other item type. On the other hand, an Orb of Alteration can be used on any of these item types if they are Magic, reforging its modifiers with new ones.

Scrolls, such as Scroll of Wisdom can be used to identify a magic, unique or rare items, maps or flasks, or a Portal Scroll can open a portal to the safety of Town, refilling our flasks and healing us in a tough boss fight or if our inventory is full and we are in need of a break.

Some league-specific currency make it to Standard after the Challenge Leagues end, for example Orb of Horizons was introduced patch 3.0 and the Harbinger Challenge League, but later it was moved to Standard making it a basic currency in the end. With Orb of Horizons we are able to reforge a map item to another of the same tier, so we can complete our Atlas Challenges easier in a Challenge League. Orb of Unmaking was introduced in Patch 3.13 and the Echoes of Atlas Expansion, and they only dropped in the related Ritual Challenge League, but was later also moved to Standard. Orb of Unmaking works similarly to an Orb of Regret, except it grants an Atlas Passive refund point instead of a Passive Skill Tree refund point.

Many currencies can have fragments, or shards dropped. They are only a part of the related currency, for example twenty Chaos Shards or Exalted Shards can be combined to a Chaos or Exalted Orb.

Currency and Item exchange rates can be checked on the official Path of Exile trading site (pathofexile.com/trade) or other third-party sites, for example poe.ninja. These rates are fluctuating continuously, depending on the amount of players in the game, and the League and Expansion running.

The crafting system in Path of Exile is very complex, these were just a few examples to make it a bit clearer, but mastering the this aspect of the game has a long learning curve to understand, just like the game itself - and it is getting more and more complex with every Expansion and Challenge League - another reason why Path of Exile does not lose from its popularity.