A Great Person, specialized in trade.
The unit is expended after performing either of these actions.
Besides constructing the special Customs House improvement which, when worked, produces loads of Gold, the Great Merchant can also journey to a city-state and perform a "Trade Mission," which produces gobs of Gold and gains 30? points of Influence with the city-state. The Great Merchant is expended when used in either of these ways.
In order to use Trade Mission, you need to enter the target City-State's territory with the Merchant.
Again, you have the choice to use it for a long-term boost of the Gold production in one city (which you should choose carefully), or use its unique ability to get a large one-time boost of Gold and Influence with a city-state. Note that the trade mission may sometimes prove dangerous, especially if the city-state is far away!
The money gained from performing a Trade Mission is calculated with the formula
350 + (50 * era_number)
where era_number is a simple serial number starting with 0 for the Ancient Era. In addition, game pacing also affects this number; Quick games modify this value by 0.67, Epic by 1.5, and Marathon by 3.
With Brave New World the formula changes to
300 + (100 * era_number)
In vanilla Civilization V and Gods & Kings, completing the Commerce policy tree doubles the Gold income from Trade Missions. With Brave New World, simply adopting the Entrepreneurship policy from the Commerce tree suffices.
"Great Merchants" provide the goods and services civilizations need to survive. They import food, medicines, and luxuries, and they export a civilization's manufactured goods. They provide the financing needed for great works to be accomplished. They create a civilization's wealth.