Page views (or hits) - Total page hits. A single person may click around & generate an average of 2-6 hits.
Unique Visits - Actual individual visits not page hits, including those who left, closed their browser, but came back again within a day (California time).
First Time Visits - Actual different people who visited in a day, not including if they visited again later in the day (California time). Can you imagine if they all came up with just a dollar for me? I could retire. But we still only have a very small handful of people who ever donate.
Returning Visits - Those who individuals left, closed their browser but came back again one or more times in a day.
On average we get 12,000 hits a day. But that's from a little over an average of 4,000 different people a day. About 800 of them visit more than once a day. Our typical visitor stops in about once every day or 2, but we have many that just visit just a couple times a week, or just every week or 2. A lot just visit on weekends.
You'd think with 12,000 average hits a day, that means only a little over 8 hits a minute, so any decent single server could handle it good enough. But that's not how the real world works. The traffic is not spread out even over the day or even over just an hour. We can get dozens of hits per minute for a few minutes that the servers have to handle without crashing, then later we may get only 1 hit every 3 minutes.
And all that traffic doesn't include attackers, relentless, persistent crawlers, spiders, pinterest hits, bandwidth theft attempts (directly trying to feed our material to their severs at our expense), & all sorts of weird stuff trying to get in or do stuff. The security alone works the servers so hard it almost needs a server of its own.
So in the real world, plan on servers about 3 times bigger what the numbers say you'll need, just to squeeze by without too many failures or crashes.