When the metrics of your site are Alexa Certified you will have access to specifically calculated visits page views and unique visitors to your site. 

“Unique Visitors” means the number of people who visited your site over the specified period of time. A user who visits the site multiple times is only count once during the time period.

Once a full page of your website is access or refresh a “Pageview” is record. The count of page views is the cumulative number of times the website pages have been update or check during the period select. Pageviews aren’t singular.

A “visit” is a one session browsing.If a visitor views another page on your website within 30 minutes of the last page view, the same visit is count. If after 30 minutes have pass since the last page view, a visitor returns to your site then it is count as a separate visit.

Pageviews are calculated when someone visits a page on your site and loads the entire page. If they are visiting the site for the first time in that interval. They will also register as a unique visitor.

If something is linking to content on your site but not actually visiting and loading. The entire page (bot, plugin, app pulling data, etc). Then this would only count as a unique visitor and not a pageview.