So here's how I understand it to work:
1) The ATV can be linked to one computer at a time. This link is done through Settings -> Computers and choose the first option to Link to iTunes. This link allows for you to sync (copy) content over to the ATV. Any content that is synced to the ATV is copied to the internal HD on the ATV.
In syncing you have 2 choices:
Automatic Sync: This will copy content for you, filling your ATV hard drive with content
Custom Sync: This will allow you to choose what content you want synced to the ATV.
Now in either case, whatever you don't sync to your ATV, will be available to stream automatically.
For example, if I have 10 movies in my iTunes library on the computer I have linked to ATV....I can sync (copy) 2 of those movies to the ATV itself. These movies are now available to the ATV even if my iTunes isn't running. However, if my iTunes is running, the ATV can see and stream the other 8 movies.
The one thing to keep in mind is that when you first setup the ATV and link it to an iTunes computer, it will be set to Auto sync and will start copying content. You can select Custom and hit apply to change this. Also click on the stop arrow in the top display in iTunes to stop the copying.
2) The ATV can access up to 5 shared iTunes libraries at once. This is also set in Settings -> Computers, choosing the second option to Add a shared library.
For shared libraries, you don't get the option to copy or sync anything to your ATV. The content will be shown in your ATV, however, the computer with the iTunes sharing needs to be on, and iTunes itself needs to be setup to share it's library across the network.
For most people, their main setup is probably going to be Linked to an iTunes computer. This is the best of both worlds (other than photos). As I understand it, photos can only be streamed via the Shared option, not the linked option. Under the linked option, you must sync photos to the ATV.
You cannot have an ATV setup to Linked and Shared to the same computer running iTunes at the same time.
This make sense? Let me know if this isn't correct.
-Kevin