Are PPC ads only shown on the search engines?
No. All the major PPC programmes have "publisher programs". This is where ads are delivered through third party sites which then share in the click revenue. Perhaps the most common of these is the Google AdSense programme. This can be a very effective revenue generator for informational sites where they don't have a product for sale in the conventional sense, and where there isn't a conflict with the type of advert being shown.
From an advertiser perspective, it can be a very good way of extending the reach of adverts beyond the scope of the search engines and onto relevant information, directory, and comparison sites which they wouldn't realistically be able to reach in any other way.
There are some negative aspects to delivering ads through so called "content networks", but in the main the benefits outweigh the drawbacks.