One-way or two-way trust

Essentially, every trust is a one-way trust. All the trusts that you create manually are one-way trusts, by default. However, you can combine two one-way trusts in opposite directions to create a two-way trust. In the scope of a two-way trust, people on both sides of the trust can access resources on both sides of the trust using their user accounts.

Microsoft recommends creating a two-way trust when your goal is to migrate accounts and resources using the Active Directory Migration Tool (ADMT). After migrating everything over, the two-way trust can be torn down.