The orb economy

Orbs are the second currency, used for crafting and barter.

Supply. Orbs drop from monsters, and bosses drop several each. The cheap ones are common; the strong ones are rare.

Tier	Orbs	Pace
Plentiful	Wisdom, Portal, Transmutation	Many each hour of play
Common	Alteration, Augmentation, Jeweller's	Dozens per hour in maps
Uncommon	Scouring, Alchemy, Regret	A few to ten per hour
Scarce	Chaos, Regal, Vaal	A few per hour
Rare	Divine, Exalted	About one an hour or less

Pricing. Orbs come from drops and player trade, so their value is set by players (Trading).

Reference values. One Chaos Orb is the usual unit. Roughly: an Alteration is a fifth, an Alchemy or Scouring a half to two-thirds, a Regal about one, a Vaal one and a half to two, a Divine about six, an Exalted about ten. The real rates are set by trade, not by the server.

What consumes them

Transmutation, Alteration and Augmentation: tens per target, especially flasks (Flask charges and affixes).
Scouring, Alchemy, Chaos, Regal: bulk rare crafting (Crafting basics).
Jeweller's: sockets, the biggest sink (Sockets).
Regret: respec (Passive points and respec).
Vaal: corruption (Corruption and the Vaal Orb).

No bound orbs. All orbs are tradeable. Quest items, Writs and Seals are bound (Pinnacles and Writs).

Mapping boosts it. Maps give far more orbs per hour than the campaign, because you kill much faster.

Watch the market. If an orb is easy to find but never used, its drop rate is lowered. If it is always used up, it is a healthy sink.