A common mistake people make when setting up a pay per click campaign in Google Adwords is to run the same campaign on both the search and content networks. Content network campaigns are quite different than search network campaigns in how they're set up, optimized, and tracked. The messaging that works best on a content network campaign may not work best in a search network campaign and vice versa. For this reason, it is best to run each campaign on only one network and optimize each campaign individually. You can even duplicate a campaign and run one version on search and a mirror of it on content to see which performs better.
If you are new to advertising on a content network, Google has a step by step tutorial on getting a content network campaign started.
