In the context of internet applications, an affiliate is someone that refers customers to providers. It is done via a unique link given to the affiliate.
For example, we are affiliated with Aweber email marketing co. Our unique code is 516565. When you click the Aweber link on our site or newsletter, the URL, or the actual link to the Aweber site, looks like this: https://www.aweber.com/easy-email.htm?id=516565 – Noticed our code at the end of that string? When you land on their site, Aweber records this referral, and when you actually sign up for services, we receive a small commission.
Tidbit: Affiliate marketing can big business for “Influencers” and sites with lots of traffic – they can generate serious dollars from referrals.