As in most games, there is money involved, in this game, you can use it to buy ships, goods, buildings and Letters of Marque. There are many ways of making money too in the game, some of which include:
For more information see Trading
Trading is a fairly simple way of making money, buy goods and sell them at a higher price.
For more information see Trade routes
Trade routes are a bit more complicated, you have to set a ship to automatically buy and sell goods but it's not as simple as it sounds because of stupid captains and pirates.
For more information see Business
Businesses are the safest way of making money, but they don't make much money until you tie them in with a trade route. They also need a permit and they're pains to get until you have all the money you need (Why you'd want businesses then anyway is a question).
For more information see Battles
Battles are one of the quickest ways to gain money but unless done properly will destroy most of your businesses, trade routes and relations with all nations.
For more information see Pirate hideouts
Destroying pirate hideouts needs powerful ships and a lot of luck butgets you quite a bit of money.
For more information see Looting towns
Looting towns can be deadly for many people but if done correctly will reap massive rewards and in certain circumstances a nation will ask you to capture a town.
For more information see Loans
At the start of the game, you will be offered loans but as the game progresses these people need money, help them to receive good rewards.