Playerunknown's Battlegrounds is a battle royale shooter published by Bluehole, based on an ARMA 2 mod built by Brendan Greene.
At the start of the game, up to 100 players parachute from an airplane onto a desolate, vaguely Eastern European island, populated entirely by guns that somebody lovingly placed inside of the multitude of abandoned homes, ruins, and military bases. Assuming you survive the depredations of the people who would like nothing more than to punch you to death within 30 seconds of landing, your only objective is to be the last person standing. You accomplish this by looting homes for weapons and equipment, finding vehicles that you can use to cover ground quickly, hunting other players, or hunkering down in a nice, quiet spot for as long as possible.
As the match progresses, an Electric Wall of Death begins to collapse inward towards a randomly selected point on the map, pushing you and 99 of your closest murder buddies progressively closer to one another. Get stuck outside the circle, and you'll slowly watch your health tick away. Get careless trying to stay inside the circle, and you'll be shot by someone that you'll never see. At least until they run over to your body and proceed to take all of your carefully looted gear for themselves. And that will happen to you, so be prepared for it. If you manage to outlast the other 99 players, your reward is a metaphorical chicken dinner, and a bunch of fake money you can use to buy fake clothes so that your avatar can look legitimately fresh and/or tight.
It's a very tense and unique multiplayer experience, and it's a lot of fun, even if it does tend to be singularly frustrating at times. The game's available for early access right now on Steam, and while it's pretty unpolished at the moment, the gameplay is solid. And you can play in teams of two-to-four people, or just go Rambo and fight your way to the top of the solo charts.
Here's a handy spreadsheet for when you want to PUBG with other SE++ people!
And here's a map detailing the best drop zones available in the game. Fair warning, it is very big, hence linking it rather than just dropping it in this post.
And here's a good guide video if you're just starting out.