It's distinguished from search-based advertising (e.g. adwords). It's basically just ordinary online advertising, and there are a great many different ad networks out there already. However, google's network has been gaining a huge amount of traction recently and took the top spot in marketshare within the last year or so, even pulling ahead of Facebook. In total these sorts of ads are worth around $2 billion a year in revenue for google but both the size of the market and google's share of it are growing at double digit rates so google's revenue from it is likely to double within the next few years.