When contextual ads work, they make lots of bloggers lots of money; when they don't, they end up on 'Bad Contextual Ads,' a photo blog that parodies the worst pairings of ads and context.
Contextual ads are meant to display ads that match keywords that appear on an individual page. Most of the time, this works out well: a tech article generates ads on where to buy a phone or computer. But in the first example above from 'Bad Contextual Ads,' however, the image of a cat vomiting is paired with an ad for "Weight Loss Without Dieting." Tragic, indeed.