In order to drive traffic to their web sites, some sites will make requests to your web site and provide their web site as the referrer (in the HTTP header). When you run a Referrers report you will then see these "spam" referrers and you might be tempted to click on them to see what they are, thus driving up traffic to their site.
FlashStats provides the ability to filter out all traffic which includes spam referrers. If any hit within a visit includes a spam referrer then the entire visit is ignored.
The generic term for the piece of client software which requests a file from a web server. The most common type of user agent is the browser. Additional types of user agents are search engine "spiders" which scour the Web looking for documents to index, and network monitors which continually monitor web sites to see if they are still running.