Pokémon GO is an enhanced reality treasure hunting game where the players walk around in the real world to reach certain points on the map where the game has stored treasures… and Pokémon. While just walking around and collecting things is one of the attractions, the players also compete (in global teams) to "take control" of bases – or, as they are called, "Gyms".
There are roughly three types of possible encounters when walking around with Pokémon GO – Pokéstops, Gyms and Pokémon. Those already visible on the map when you turn the game on, are the Gyms and Pokéstops. Both of these are all based around noticable landmarks in the real world, like a church, a monument or a statue. Pokéstops are most common. Walking up to a Pokéstop and flicking the screen rewards the player with Pokéballs, or sometimes other useful items such as healing potions, eggs or revival crystals. A "Gym" is essentially a base that can be taken.
Pokémon do not show up on the screen until the player is very close, although the game gives hints on what kind of Pokémon are close by. When a Pokémon shows up, a tap on the screen will start the catching seqence, where the player flicks his Pokéballs against the creature (which shows up on the mobile device's camera).
As the player checks Pokéstops and catches Pokémon, he gains experience points. At a certain level, the player can start entering the Gyms, and partake in the battle to control them. The player is asked to join one of three teams, represented by Yellow, Red or Blue. All Gyms are coloured after what team currently controls it. A player can use his Pokémon(s) to fight the current Pokémon(s) on the base, regardless of whether it's a friendly or enemy base, but the rules differ.
When fighting a friendly base, the player challenges the Pokémon(s) there with one of his creatures. If he defeats any of them, the base gains points. The base can thus level up: A level 1 base has room for only one (1) Pokémon, a level 2 base has room for two, and so on, although each player can only keep one of his Pokémons och eash base (Gym). So the more a player challenges his own team's base, the stronger it becomes, as he can create room for more Pokémons to defend it.
When fighting an enemy base, the player challenges the Pokémons there with six (6) of his creatures. If he defeats all, he has won the base, and it returns to neutral level 1. The player can then claim the base by placing one (1) of his own Pokémon there (this Pokémon will remain there until kicked out). However, if he defeats at least a few of the enemy Pokémon but not all, the base loses points, and if it levels down, the lowest Pokémon is kicked out of the base.
If a player repeatedely attacks a base, he can thus chip off the Pokémons there and will eventually be attacking with six Pokémon against the last one. This allows a lone player with several slightly weaker Pokémons (but a lot of healing potions) to actually defeat a base, even if the Pokémon(s) there are a bit stronger.
When caught, Pokémons are assigned a semi-random strength, slightly depending on the level of the player. Catching a Pokémon also gains the player food for that Pokémon type. A Pokémon can be powered up, or evolved, using food for its type. Duplicate Pokémon can be turned in for extra food, to save up to evolve the strongest Pokémon of that type the player has.