How is PageRank calculated?

To calculate the PageRank for a page, all of its inbound links are taken into account. These are links from within the site and links from outside the site.
  PR(A) = (1-d) + d(PR(t1)/C(t1) + ... + PR(tn)/C(tn))
That's the equation that calculates a page's Rank. It's the original one that was published when rank was being developed, and it is probable that Google uses a variation of it but they aren't telling us what it is. It doesn't matter though, as this equation is good enough.
In the equation 't1 - tn' are pages linking to page A, 'C' is the number of outbound links that a page has and 'd' is a damping factor, usually set to 0.85.
We can think of it in a simpler way:-
a page's Page Rank = 0.15 + 0.85 * (a "share" of the PageRank of every page that links to it)
"share" = the linking page's Rank divided by the number of outbound links on the page.
Step 1: Calculate page A's rank from the value of its inbound links
  Page A now has a new Page Rank value. The calculation used the value of the inbound link from page B. But page B has an inbound link (from page A) and its new value hasn't been worked out yet, so page A's new PageRank value is based on inaccurate data and can't be accurate.
Step 2: Calculate page B's Rank from the value of its inbound links
  Page B now has a new Page value, but it can't be accurate because the calculation used the new Page Rank value of the inbound link from page A, which is inaccurate.
It's a Catch 22 situation. We can't work out A's Rank until we know B's Page Rank, and we can't work out B's Rank until we know A's Page Rank.