This document discusses online advertising. It defines advertising and describes different types of online ads like display ads, text ads, pay-per-click ads, email ads, mobile ads, and social media ads. It explains reasons for advertising online like low costs, measurability, ability to rapidly update ads, and ability to target global audiences. Challenges of online ads include banner blindness, ad-blocking, lack of trust in advertisers, privacy concerns, and irritating ads. Repetition is key to advertising success while behavioral targeting allows using user data to deliver more targeted ads.