ATP, I want to know, how does this ranking work?
The Association of tennis (ATP) rankings are currently the world benchmark to say that a player is world number 1 or 10 based on their points scored in the last one year. ATP has it origins from a trade union in 1972 to protect the interest of male tennis players. This is a standardized system that ensures fairness and the quality of players who play in the top world circuits. For women, there is the WTA women's Tennis Association rankings.
Which factors are considered for calculation?
The rules have been improvised regularly, in general, the ATP score reflects a players performance in the last 18 tournaments (Grand Slams-4, ATP-WTM 1000 -8, previous ATP final count and best of 6 from non-mandatory ATP tournaments like Davis cup etc).
In all these, the Grand Slam (Australian open + French opne+ Wimbledon+ US open) is very important. The player who qualifies for the Grand Slam gets 25 points but the winner gets 2000 points!
What is the importance of ATP ranking?
This is crucial to the reputation of the top players at the Global platform. Their skill, performance-based score is used to decide who is given a ticket to play in the world's premier tennis tournaments. Their chance at the prize money ( 3.7 Million dollars for US open winner 2017) and the endorsements etc all depend on their ATP ranking indirectly.
How does it affect the participation in major tennis tournaments?
The top players find time to keep playing the qualifying matches and also for their country ( Davis cup), they cannot afford to miss a grand slam event wherein they potential can start with a zero score. If a player is injured, he can write and request a 3-month ranking protection.(http://www.atpworldtour.com/en/rankings/rankings-faq)
Right now Rafael Nadal is ATP/world:1 with 9760 points with Roger Federer at the ATP/world:2 with 9605 points.