This document provides an overview of online digital advertising. It discusses what online advertising is, how ads are served, metrics for analyzing ads, opportunities in social media, and areas of interest for further exploration. The key aspects of online advertising covered are contextual targeting of ads, behavioral targeting based on users' online behaviors, and audience targeting which profiles audiences in more depth than aggregate compositions. Metrics for analyzing ads include ad server data, cost per action, engagement, impressions, clicks, and time spent on sites.