Microsoft Knowledge Base Article 257338 contains the following summary:
The File Replication Service (FRS) is a multi-threaded,
replication engine that
replaces the LMREPL service in Microsoft Windows NT 3.x and 4.0.
Microsoft Windows 2000 domain controllers and servers use FRS to
replicate system policy and
login scripts for Windows 2000 and down-level clients.
FRS can also replicate content between Windows 2000 servers hosting the same fault-tolerant DFS roots or child node replicas.
This article describes troubleshooting steps to use on Windows 2000 domain controllers that are missing netlogon and sysvol shares.