Sam Dean at FiveThirtyEight:
Uniques are what most people mean when they talk about a website’s traffic. Show up once and you count as one unique visitor — show up again in the same month, or even visit the site every day in that month, and you still count as one unique visitor (or at least that’s the idea). Uniques are the big-picture number — the Nielsen rating, the Blue Book value, the GDP — that’s supposed to show how well a website is doing. People used to talk about pageviews, a simple count of how many pages were loaded over a certain amount of time. But uniques have taken over, because uniques measure people, not pages. Advertisers care about the former when they’re planning an ad buy.
If uniques are people, how do 4 million, or 125 million, or 253 million people go missing? In an age when we assume our phones and laptops are tracking our every move, taking an actual head count of how many people go to a website is still almost impossible. There’s a blind spot at the center of the panopticon, and it’s roughly the size and shape of a cookie.
If you're interested in web tracking and advertising, Sam's piece will make for a great read.