Everything in tennis is about the
world rankings which is a 52-week rolling calendar. Each tournament is allocated a number of points.
For Grand Slams the winner gets 2000 points (there are 4 of these)
For Masters the winner gets 1000 points (there are 9 of these)
For ATP 500 the winner gets 500 points
and so on down the food chain.
Whatever points you accumulate you get to hold on to for one year, meaning in one years time, you have to 'defend' these points.
For example last year Djokovic won Australia, he's defending 2000 points this week. Nadal did not play Australia last year, he's defending 0 points.
So entry into this Australian Open tournament is simply the top 104 players in the rankings + 8 wildcards (determined by the organizers) + 16 qualifers (determined by a separate 128 man tournament)
The tournament you saw in the November was the year-end championships. The top 8 point accumulators in the calendar year 2013 get invited to a special tournament in London.