So what is Ingress?
Ingress is a new (actually still in beta testing) Massive Multiplayer Online (MMO) game. But unlike many other MMO games, like say World of Warcraft, Ingress incorporates a geography element something like Geocaching where you have to physically go to locations to play the game.
The game is available through Google Play for Android devices. https://play.google.com/store/apps/details?id=com.nianticproject.ingress. The game can only be really played with your Android versus your laptop or PC. That being said there is some very useful information online.
Ingress Intel Maps - This is a great tool to find nearby portals and doing reconnaissance before you head out. You can see the number and status of the resonators and any mods a portal may have.
Niantic Project - This site is interesting to say the least. Each month it has a different theme with online "riddles" which if you solve you can get codes for items.
So you want to play?
Right now that is easier said than done. The game is still in a closed beta stage. You can feel free to request an invitation to play at either ingress.com or nianticproject.com. Be prepared to wait. The selection seems to be rather random. I applied shortly after Thanksgiving 2012 and received my invitation just after New Years. But others I know have applied before me and still haven't received received their invitation.
There is another way, you can get an invite from someone who plays. Now. I haven't figured out how current players earn invites to give out. I haven't earned any yet, so don't write asking ;)