My website's Google Analytics shows me 100,000 unique visitors for a time period of 12 full months. Does that means that these are actually 100,000 different people that visited my website in a year?
No, unfortunately it's not literally 100,000 different people. More accurately, it's 100,000 different cookie values. A user can visit your site for the very first time and be counted with a first unique visit, but if they clear their cookies, or log in from a different computer, or do private browsing, then a subsequent visit would be "unique" again. So technically, they have generated two unique visits, but it's the same person.