The Playzone, also known as "The Circle," is the area where players must remain during the game. If outside of the playzone, players take damage. This forces players closer to each other as the game progresses in order to speed up the game.
All playzone / circle timings are for the PC version of the game.
The time between entering the plane and being able to jump is about 10 seconds.
The time between entering the plane and seeing the first circle is 120 seconds (2 minutes).
Timings for each circle are below (circle size may be off by up to 10 meters).
When the blue circle meets the final circle 8, 180 seconds (3 minutes) will pass before a 15 second countdown begins until the circle begins to shrink to nothing. Damage during the final phase of the circle shrinking to nothing is 11%.
As of 10/23/17, circle damage and survival times are not fixed, but variable based on distance. Circle got buffed and each circle after the first now does more damage the further outside the playzone you are.
Circle 6-8, do not help your downed mates outside the play area. Circle 5, you can survive for 2 ticks after being helped up, so you'd have to be close to to entering the circle. Circle 4, you can survive for 5 seconds - the player helping you will take 10% damage while helping you. Circle 1-4, help your buddies.
Damage ticks are sent to every player at the same time. (The first tick can happen anywhere from 0-1.) Every circle ticks damage each second. There's not a 9th circle, once the 8th circle hits the white circle - it simply pauses and then closes down completely. (Damage and everything is the same.) The 6th circle is the last circle where you can heal and run in. To win in the last circle if you can't see the last enemy, take a hit of damage from the circle, and pre-use your med kit 2 seconds before the last circle hits you.
Sanhok is the first map where BATTLEGROUNDS implemented a dynamic circle. This dynamic circle sees how many players are alive then will calculate if the blue zone should move faster or slower throughout the match.
In the Battle Royale movie, it's a grid that seals off players. But in ARMA, PLAYERUNKNOWN couldn't code a grid, so the 'circle' became the new Meta.
↑ Twitter announcement from @PUBG_help, describing the deployment of PC 1.0 Update #10.