A lot of questions seem to constantly pop up about the rankings. Here are some general guidelines:
You can go to
http://atptennis.com and look up any player. Click in their playing activity to view their results for any year in their career. You'll see each match in every tournament and how many points they were awarded for each result. As a result, you can calculate for yourself how many points they are defending in each upcoming tournament.
In general, for Grand Slams, pts are awarded as following:
1st rd - 5pts (20 if you made it through qualifying)
2nd rd - 35pts
3rd rd - 75pts
4th rd - 150pts
QF - 250pts
SF - 450pts
F - 700pts
W - 1000pts
For masters series, approximately half the pts are awarded, with a few slight exceptions in the early rds.
For smaller tournaments, pts can vary, with some of the bigger events such as Dubai and Barcelona awarding 300pts to the winner, with smaller events like Newport awarding 175pts, and others anywhere in between. The Olympics will award 400pts to the winner this year.
A player's rankings is dependent upon how many pts they've acquired over the last 52 weeks, counting the 4 slams, 9 masters series, and best 5 other tournaments. For lower ranked players who didn't automatically qualify for masters series or slams, they can use more smaller events, challengers, futures, etc...to make up their best 18 results.
For those who qualified for the year end championships, they get a free 19th tournament for extra pts, with 100pts being awarded for each round robin win, 200 pts for a SF win, and 250pts for a final win. Thus you can win either 650 or 750 for winning the entire tournament. The masters cup points are the only points that don't hold for 52 weeks though, as they go off the books after only 51 (or 50) weeks so that going in to the masters cup, the points race matches up with the rankings. Federer currently has 650 pts from last year's masters cup to Nadal's 200, so it seems very probable that even if Federer defends a lot of his points from last for the remainder of the season, after MS Paris, Nadal will get to be #1 going into the masters cup.