Visits are sessions. So, when John lands up on a website, looks through few pages, submits a form, clicks on a link and then moves on to another website. John will be regarded as a single visitor.
Now, he takes a break, calls up a friend and after 10 minutes, he returns to view the same website, he will still be regarded as a single visitor.
However, he now takes a break for 30 minutes and then views the same website again, John will now be classified as a new visitor.
A visit is defined as a series of interactions without a 30 minute break or a continuous activity spanning 12 hours. However, different analytics tools can have their own session window. So while Google Analytics may follow a 30 minute time-out limit, the same number may not hold true for another web analytics tool.
Unique Visitors are visits during a defined time period. So, in the above example, if John returns to view the website on day 2, he will be counted as new visitor for day 2. However, he will still be a single visitor during the time period Day 1 + Day 2.
Pageviews are the number of times a specific page is viewed/ loaded. However, any interactions within a page that does not lead to a new page load should not be counted as a page view. For e.g.: Button clicks should not be attributed as pageviews for a specific page. It should be classified as event.
Instances is a metric unique to Adobe Analytics (Omniture SiteCatalyst). It is the number of times a specific variable is defined. So, each time Adobe Analytics find a value in the variable, it increments the instances by one.
Instances = Pageviews + Events
Instances = s.t () + s.tl ()