Visitors are unidentified individuals who visit your website through a search engine, referral or direct visit. These visitors can provide anonymous data to you, in the form of page views, time on page (duration) and other metrics; yet, these visitors are unreachable by marketing people. On the other hand, prospects are visitors who have identified themselves, for example, by filling out a form. Once visitors are converted into prospects, marketers can gain an immense amount of insight into prospects’ level of interest in a product or service and collect qualifying information.
Visitor cookie for unidentified individual
As soon as someone starts browsing your website, a visitor cookie gets created in their browser by the Pardot tracking code installed on your site. The site uses cookies to recognize visitors and record their behavior in the Pardot database. If someone is using multiple browsers to visit your site, in such cases he will have unique visitor cookie for each browser.

Visitor Cookie
Visitor Id is nothing but the visitor’s record unique identifier in Pardot, as shown in the following screenshot

Visitor’s ID
Visitor object stores the basic information about a visitor, for example, Number of page views by the visitor, Source, Visitor’s IP address, Visitor’s Hostname, First Page View and Last Page View, etc. Whereas Visits object stores the basic information about visitor’s or prospect individual visit (only page visit), for example, Number of page views by the visitor, Length of each visit, Time of first page view for each visit and Time of last page view for each visit, etc.

Visits point to unique Visitor
You may see the multiple lines into the visits table, depend on visitor’s page view activity. It means one visitor can do multiple visits to your site. All this information is tracked by Visitor Cookie. A visit record can also associate with a prospect record.
Association of a visitor cookie to a prospect record
Pardot offers different ways, for example, Form, Form Handlers or Landing Pages, etc. to convert anonymous visitors to prospects. A prospect is a known person who has provided their email address by submitting one of the Forms or Landing Pages on your website. The prospect’s email address is a unique identifier (if Allowing Multiple Prospects with the Same Email Address is not enabled), which allows Pardot to track prospect’s activity moving forward.