Within Sea of Thieves, there are three main Trading Companies. Each Trading Company specializes in a given type of item or good, and each will give you Quests, known as Voyages, for you to go on in order to earn both Money and more reputation with their given company. The Voyages the Trading Companies will send you on with vary from each company to the next, ranging from having to find buried treasure to having to defeat Skeleton Captains to even having to race to find certain types of Animals.
With each Trading Company, you will earn more and more reputation with each of them as you accomplish more and more Voyages. The more Quests you complete and the higher your reputation with each Trading Company rises, the more Titles you will earn with them and the more rewards you will receive. Eventually, you will even be able to buy special Items from stores that are in association with each of the Trading Companies, one that are actually locked until you reach a certain rank with each of them.
Below are the three Trading Companies you will find in Sea of Thieves, as well as a description of each:
Gold Hoarders: This Trading Company seriously values gold. They will send you on Voyages to find and obtain certain Treasure Chests, the same they will only have the keys to open.
Order of Souls: Oddly enough, this Trading Company focuses on the occult value of the dead. Because of this, they will send you on Voyages to defeat Skeleton Captains and obtain various skulls.
Merchant Alliance: This Trading Company focuses on the delivery of goods, like Animals and other Trade Goods. However, they are extremely into prompt delivery, forcing you to work with strict deadlines.