What is the difference between Visitor and Unique Visitor?

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:

• Page Views = total number of pages viewed by all people.
• Visitors = total number of browsing sessions by all people.
• Unique Visitors = total number of people who visited.

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.