Affiliate marketing is simply a method of marketing in which you can earn a commission or fee for promoting the products or services of others.
You, the Affiliate, agree to promote those products and/or services through your website, blog, email campaigns, or any other method permitted, and any sales you make will earn you a set fee / commission or an agreed percentage payment.
Affiliate marketing, when done correctly, can be very profitable; many people make a good living simply by promoting the products of others.
You won't have to worry about creating your own product as an Affiliate Marketer, and you won't have to deal with customers or customer support issues.
To become an Affiliate Marketer, you'll need some basic internet marketing knowledge and, more importantly, "the desire to succeed."