Revenue
The purpose of any business is to generate revenue, which hopefully will result in profits.
There are two fundamental areas revenue comes from online: 1. Directly from the site: selling a product, subscription or advertising. 2. Via a third party: sending traffic to them or selling their products in exchange for commission.
Alternatively, your site could be used to drive potential customers to your offline business.
With the right tools, selling and taking payment on your site is easy to set up. To find out how to sell your products/services and take payments, go to making sales.