Definition - Traditional affiliate marketing is when a retailer or service company pays a referral fee (usually a Cost Per Acquisition or % of sale)  for a sale or lead.

Marketing - Marketers use affiliates to drive additional sales/leads they wouldn’t have gotten through traditional marketing. Affiliates tend to be publishers whose user base utilize them to make buying decision as first time users or bottom of the funnel activity.

Optimization - When utilizing affiliates make sure the traffic is accretive  and not overlapping or cannibalizing sales that would have occurred without the affiliate.