Picture this, we can use Sharepoint in multy-forests environment, with the support needed, there are two main scenarios: Resource forest and Multi-login forest.
When we have a one way trust relation between the forests, and we can transport the users from one forest (resource) to be used in another (corp.).
We have two way trust between forests, this means that we can , from a Sharepoint point of view, use the users from both forests without concern on their location.
There are a lot more information on the article that is relevant for understanding the concept.
You can find this article here.