This is the next SEO technique that Google appears to be going after (or has gone after big style in the last couple of months). A blog network is a group of blogs – normally bog standard wordpress installs, with basic wordpress templates and lots of content. Generally this content is generated explicitly for the purposes of creating links back to a website. Read more
For some time there has been a growing number of content farms that are able to produce a lot of content for next to nothing, the benefit of this is that they are able to suck up lots of traffic and serve it with plenty of adverts.