The affiliate functionality creates a unique URL which allows you to keep track orders placed under that link. 
Its really easy to set up:
- Go to Promotions > Affiliate 
- Click "Add new"

- Complete all fields and ensure that the active option is ticked 
- Click "save and continue edit"
- You will be provided with an Affiliate URL to use 

You will then see 2 additional tabs "affiliate customers" and "affiliate orders" these tabs with hold information for you of all customers registered and orders placed using the affiliate API