Referral spam is the placement of domain names into the referral list in Google Analytics and other web analytics tools. There are two types of referral spam:
- Crawler-based, where a robot crawls your site and passes the spammers referrer. In this case there was a visit to your site, but not by a real person.
- Spoof-based where the spammer calls Google Analytics and passes the Site ID for your web site. The site ID can be randomly generated or obtained by crawling your site.