As with every aspect of Google Adwords, the networks available to advertise on (the search network and the content network) are vast and perhaps complicated to understand.
Whilst learning for my exam, I find it difficult to fully understand the concept of contextually targeted campaigns.
To start with a brief description, Google determines what adverts are shown on theme fundamentally.
As an advertiser, you should have 50-60 keywords that relate to a very distinctive theme within your advert, and it is that ad group with its theme of keywords that will trigger your ad on a webpage within the content network.
Google will then implement adverts signed up to appear on the content network, and put them onto relevant pages accordingly.
The content network is vast in comparison to the search network, hence why millions more impressions appear on the content network.
However, an advertiser should fully expect to see a much lower CTR for an ad on the content network against adverts advertising on the search network.