What is a refferral code?

A string of letters and digits used to identify an associate website to another site it links to.

Example:

You enter an agreement with Amazon that you will link to Amazon books on your blog. In return you get some money per click or per book bought by people coming from your blog.

For this to work, Amazon needs to know which of their visitors came from your blog, so you get a referral code: Your ID in the Amazon system.

Referral codes are used by adding them to the links on your site, so an example link "Amazon.com/book1" could be turned into "Amazon.com/book1/ref=abc", where "abc" is your referral code.

Someone clicking the link on your blog would then use the special link with the referral code that tells Amazon they came from you.