Some websites use more than one domain or several subdomains to serve content to visitors. You could for instance have most of your website’s pages located on www.domain.com, you’re your support forum is located on a sub-domain, support.domain.com. Google Analytics considers both of these domains as different entities and will not track them automatically. This means that whenever you visit both domains, Google Analytics will consider you to have made two visits, instead of just one. The same problem occurs with two entirely different domain names (www.domain.com and www.domain123.com).