Microsoft Knowledge Base Article 304491 contains the following summary:
This article describes the Conditional Forwarding feature that is included in Windows Server 2003. A Windows Server 2003-based DNS server can use conditional forwarding to forward queries to other DNS servers based on the DNS domain names that are in the queries. For example, with conditional forwarding, a Windows Server 2003-based DNS server could be configured to forward all of the queries it receives for names that end with sales.microsoft.com to a specific DNS server's IP address, or to the IP addresses of multiple DNS servers. Only Windows Server 2003-based servers can be configured to do this forwarding, but the servers that are running DNS and that receive these forwarded queries can be running any version of DNS.