Hi Mike,
I think your question is how to track the same user across multiple devices.
Ibrahim and Christy explained well how to track visitors through different sources.
I have some point here to explain how can we track the same user accross multiple domain as the same person. Currently we dont have any better possible techniques I guess, because the Google Analytics team itself are in the work shop to get this done

.
Universal Analytics is designed to help track users across multiple devices, giving a much better picture of user behavior as an individual and not a series of disparate sessions.
Now we can shift from a visit centric view (current Google Analytics) to a real user centric view (Universal Analytics). In the past, we only measured visits by a device, but did not have data about who this person actually was. Now Universal Google Analytics now allows you to track users across devices, mediums, platforms and more!
Suppose the first visit is driven by an email marketing on a tablet. The second visit is on a desktop computer from a direct visit by the same user. In traditional Google Analytics, this would be treated as two different visitors and Direct visits would get the credit for the sale. But that was the old Google Analytics. This ability to track the same user across multiple devices can be a game changer for some businesses.
A ‘standard’ GA relies on cookies to join together the dots, and uses ‘sessions’ as its main building block, ‘universal’ can be all server based, and attempts to make ‘users’ its main building block. You use an anonymous ID to link together all data related to a user.
No longer will visitors to your site receive four first party cookies from GA, but one. This one cookie will contain a unique visitor ID that will enable your website to read and store it when someone logs in, for example. This new “measurement protocol” then allows you connect your information about the anonymous visitor ID in Google Analytics. If that visitor’s ID then logs in again on your site, but this time from an IPhone, Google Analytics understands that it’s the same visitor using your site, but on multiple devices. I think its time for us to say good bye to visit-centric analytics, and hello visitor centric analytics, coz UA is in Beta version now (only for limited users)

.