A Visitor represents a new browser session on your site. Visitors are associated with properties and can be opted-in to ShopMessage with their Facebook account. People who come to your site on multiple devices may have multiple visitor profiles, but ShopMessage can know about how different visitor profiles map to the same person depending on when and how they opt in.
Visitor profiles are generated automatically with the JavaScript SDK in the browser, and ShopMessage provides various functions on the visitor profile for you to interact with.