Paid social media refers to all forms of marketing use of social media where advertisers pay for ad placement.
More common forms of paid social media are:
YouTube sponsored videos (if we consider YouTube as a social media platform).
Paid social media may use traditional display formats or native advertising formats.
How agencies and advertisers use paid social media:
Image credit "Paid Social Media Advertising - Industry update and best practices 2013" VIzu - Nielsen.