Microsoft Knowledge Base Article 308195 contains the following summary:
This article describes how to establish a trust
relationship between a Microsoft Windows NT 4.0-based domain and a Windows
Creating a trust with a Windows NT-based domain is essentially using the Windows NT trust model in a Windows 2000-based environment. Windows NT trusts are one-way trusts between a "trusting" domain and a "trusted" domain. For example, if you have a Windows 2000-based domain whose users want to gain access to resources that are stored in a Windows NT-based domain, you must create a trust relationship in which the Windows NT-based domain trusts the users from the Windows 2000-based domain. In this case, the Windows NT-based domain is the trusting domain and the Windows 2000-based domain is the trusted domain.