As the title says; What is the difference between Visitor and Unique Visitor?
A visitor is a user with a cookie for one browser, as long as that cookie is valid, isn't that unique enough? If not, how is unique visitor calculated?
Visitor: visitor who already has visited your website
Unique visitor: visitor who has never visited your website before the event of becoming a visitor (for a date range)
Indeed, it's about cookie and complicated, all is explained on Google's support.
There are three main metrics in Google Analytics terminology: Page Views, Visitors and Unique Visitors. They are each slightly different.
First there is the concept of a Visit. This is essentially a browsing session. Visits last until there is 30 minutes of inactivity, or at midnight. (A new visit also occurs if the user visits the site via a different "campaign" but I'll assume you're not using those.)
Each visit consists of a number of page views. So if a user starts at the home page, goes to the About page then leaves the site, that is 2 page views in one visit. If they come back a few hours later and do the same thing, it then counts as 2 visits but 1 unique visitor, and a total of 4 page views. This is explained in more detail here.
In summary:
EDIT:
Upon each page view, Google Analytics checks for the existence of its cookie with the _utma identifier.
If that value is not set or present, the generated visit by the visitor is counted as an unique visitor. If that cookie is present (and the subsequent page views by that visitor) are then counted as non-unique visitors.
END EDIT
Within Google Analytics, Visitors and Unique Visitors refers to the count of visitor ID's the site got during the pre-selected date range.
Unique Visitors are identified by the _utma cookie (and its unique visitor ID). So, let's say the _utma cookie gets deleted or the person changes computers or browsers, they won't be tracked as the same visitor.
For example:
Someone visits the site 10 times in the selected date range. That equals to 1 Unique Visitor and 10 visits.
or
Someone visits the site 5 times with browser A and 5 times with browser B in the selected date-range. That equals to 2 Unique Visitors and 10 Visits.
Visitor : If user come again and again from same IP Address is known as visitor.
Unique Visitor : If user come first time from any IP Address is known as Unique visitor.
There is a lot of misinformation and confusion in the previous answers to this question - partly caused by lack of clarity on the part of Google - and none of the previous answers is correct.
To clarify two points: - contrary to what Zistoloen states, neither 'visitors' nor 'unique visitors' has any relation to whether the person has visited the site before. This comes into play with 'New Visits'. (Each new visit represents a new visitor.) - contrary to what DisgruntledGoat says, the 3 fundamental metrics in GA terminology are not Page Views, Visits, Unique Visitors (not Page Views, Visitors, Unique Visitors).
If you substitute the term 'Visit' for 'Visitor' in DisgruntledGoat's answer, the answer is correct.
This still doesn't answer the original question. Google explains how unique visitors are calculated here but this doesn't clarify what the 'Visitor' metric available in Custom Reports means.
One key thing to note is that in some cases the 'Visitor' metric provides non-de-duplicated visitor counts, unlike 'Unique Visitors'. Using this GA Custom Report you can see that at the month level, 'Visitors' and 'Unique Visitors' are the same. If you drill down to the day level, they are still the same for each day, but - assuming you have at least some visitors who make 2 or more visits during the period - now the total for 'Visitors' is much higher. It is the non-de-duplicated total of the daily unique visitors, overstating the number of unique visitors during the period. Meanwhile, the 'Unique Visitors' still matches the number of 'Unique Visitors' in the Audience Overview report.
Based on the above, it is pretty clear that Unique Visitors should be used instead of Visitors. However, applying some segments can lead to Visitor counts that are lower than Unique Visitors. I can't find any source that explains that.