Ingress is a massively-multiplayer augmented reality mobile game. It is different from most games in that performing actions requires you to visit real-world locations. Ingress is the first game developed by Niantic, Inc. (originally part of Google). Many of the concepts developed for Ingress were later applied to Niantic’s second game, Pokémon Go.
Below is an official Niantic video that describes the basics of Ingress play. A text explanation is provided below the video if you prefer.
The core gameplay mechanic is control of portals. Portals are real-world locations; to interact with a portal, Ingress players (called agents) must physically go there and view it in the Ingress application on a mobile device (referred to as the scanner). Portals are typically significant man-made landmarks: public art, memorials, notable buildings, etc.
There are two factions in the game: the Enlightened and the Resistance. These two factions are represented by colors: green for the Enlightened and blue for the Resistance. A new agent must select which faction to join. Portals can be controlled by either faction; the portal’s color indicates its faction alignment. A grey portal is neutral, meaning it is not controlled by either faction.
One of the main purposes of portals is to provide you with items for your inventory. Items are obtained by hacking the portal: simply tap the portal in the scanner, then tap the Hack button. One of the items you can hack is called a resonator. These can be used to capture neutral portals. Deploying a single resonator on a neutral portal captures it for your faction. You can further strengthen the portal by deploying additional resonators, up to eight total. A portal with eight resonators deployed on it is said to be fully resonated or fully powered.
If you encounter an enemy portal, you can attack it using XMP bursters. Doing so damages its resonators, but it will also attack you back. If you destroy all the resonators on an enemy portal, it will become neutral and turn grey. You can then capture it by deploying a resonator. XMP bursters are obtained by hacking portals, just like resonators.
Once a portal belongs to your faction and is fully resonated, it is ready to be linked. To link a portal, you need its key, which you can obtain by hacking it. You cannot use the key at the portal from which it was hacked. Instead, you must find another fully resonated portal belonging to your faction. Tap that portal in your scanner and select the Link button. You will then have the option to use your key from the first portal to link them together. This will show as a beam of light between the two portals. Linking portals strengthens them, and is also the first step to capturing territory in the game.
Once you have linked two portals, you can find a third portal and link it to each of the other two. The triangle formed by these three links will fill in; this is called a control field, and is how agents capture territory for their faction.
For more information, see the linked articles above, or view the table of contents.