An ARG is an "Alternate Reality Game". It is one in which the occasion or information can occasionally link to real life pages, created for that specific ARG, twitter accounts, e-mails, and using web pages and books to find out information and thus figure out the clues. We have been informed that AOH may be seeing quite a few ARGs in the future and this page will have a list of them.
You can read more about ARGs here:
Wikipedia ARG Article
And see various examples of well known ARGs at:
Unfiction Website
ARGs can use many types of devices and a WIDE variety of knowledge. A recent one for the release of another game involved Morse code, Bible verses, ciphers of all sorts, decoding using webpages, and even stenography (the hiding of files within .jpegs, simplified) for certain pictures. There's no telling what the AOH ARGs may bring on, but don't discount any bit of knowledge. As they become apparent to be needed, I'll try to include lists of potential tools to use here.
A Mysterious Tablet (July 2012)
A mystery begins when a tablet is purchased at Auction
Google Docs - The Orb Experiment (18 July 2012)
WARNING!!! This is a work in progress. Extreme spoilers with solutions to puzzles. You have been warned!
Orb Experiment (two weeks later) WARNING!!! All sections lead to possible spoilers
Sylvia sent a vision to Jostinian with information