Is an android mobile phone app that was released Nov 15, 2012 by NianticLabs@Google. It can be downloaded from the play store here.
On July 2014 it was also released for iOS. You can find it in the apple store
Players must choose a faction to join: Enlightened or Resistance
(a change in faction is possible via this link but you lose the progress done and keep only badges won and stats).
When hacking portals occasionally you will obtain Media (videos, images documents). Some of them contain hidden info others are just story related. The ones found so far can be seen here
Occasionally, Niantic leaves hidden codes in messages and bits of media. These codes can be entered into the scanner or on the Intel page and redeemed for AP, XM, and other items.
More info can be found here
As anything player made it may contain errors. use them at your own responsibility and feel free to add your own to the list
The app used to require users to acquire an activation code, during the closed beta, usually by signing up for the wait-list at the Ingress website or from the Niantic Project website. For the sake of record this section is still presented here)
You would receive an email like this after some time (this has been months in some cases)
There were other sources for these codes as well: