Players walk around real-world locations catching Pokemon which appear on their screens
How the game works at first
Once a player has downloaded the app, it's time to build a character
Players can choose things like clothing and accessories
Now you're ready to start hunting Pokemon monsters
Players walk around real-world locations visiting Pokestops and Gyms
Pokestops and Gyms are normally at real-world locations, including local landmarks
When Pokemon appear players can capture them using a Pokeball
At Pokestops you can also refuel on Pokeballs and other items
You can use "incense" to attract creatures towards you
Deploying a "lure" module attracts them towards a particular Pokestop, so it also helps other Pokemon hunters
How the game works after level five
Once you get past level five you are becoming a more experienced "trainer" and can start battling for Pokemon Gyms
You must choose a team - one of three, blue, red or yellow, Mystic, Valour or Instinct
You can team up with other players on the same team to win and strengthen Gyms
You've probably now caught enough characters to start trading in lower value Pokemon
Pokemon have "combat power" - the higher the better for taking over Gyms
Catching and evolving Pokemon earns you experience points, helping you reach the next level
Lucky eggs, which you can pick up at Pokestops, double your points
At level 12 you get access to a new kind of ball - a "Great Ball". these give you more power to catch Pokemon, in theory
Pokemon locations are key to the game - and part of the mystery
The mystery of the locations is a big part of many people's current obsession with the augmented reality game. So how are Pokemon placed on the map?
Much can be learned from similar GPS-based software, which may explain how Pokemon locations are decided.
A good reason for good maps
The idea of tech-meets-real-world has a history of blowing people's minds.
When Google Earth launched in 2005, the number of people looking at their home from space for the first time broke the company's servers.
The man behind that, John Hanke, was in charge of running the software at the heart of Pokemon Go. "A lot of us worked on Google Maps and Google Earth for many, many years, so we want the mapping to be good," he told Mashable.
The game is constantly changing thanks to new ways of using mapping data
Jo Reid is the managing director of Calvium Ltd - a mobile app developer, specialising in GPS and location-based apps. She reckons nostalgia combined with a generation of gamers who've grown up with a smartphone is behind the current craze.
"This just seems like a winning formula... and seems to have pushed it over the edge."
Pokemon Go is more than likely using "a combination of crowd-sourcing and knowledge about the local environment".
She explains that developers will have "mapped on two dimensions" - one being the base map, so the game knows where you are, and another pre-loaded with hotspots in which different types of Pokemon appear.
"If they've got a list of Pokemon types that are associated with those areas they can create an algorithm to spawn those Pokemon there."
This essentially means water-based characters will be in ponds, lakes or by the coast and ghost-type Pokemon will appear at graveyards or cemeteries.
Aim for tourist areas to catch 'em all
It looks like Nintendo has taken the bulk of its mapping information from Ingress, a previous game from Niantic.
"Prioritising public attractions and generally known public spaces is something the guys will have developed from the Ingress games and will be applying those lessons to this."
Pokemon Go's creators have also gathered data about what the game's users have done at certain points, according to Jo's analysis.
"They can record that activity," she explains, which is then fed back into the data that is driving future locations.
Based on the theory that certain areas will have been programmed as hotspots, and that the app will react to busy areas, tourist attractions are prime hunting grounds.
There are many stories about Pokemon hunters getting into sticky situations
Meet the 21-year-old who carried on playing after being stabbed
Michael Baker was playing the game in the US state of Oregon at about one in the morning.