Microsoft Knowledge Base Article 324175 contains the following summary:

The System Volume (Sysvol) is a shared directory that stores the server copy of the domain's public files that must be shared for common access and replication throughout a domain. The Sysvol folder on a domain controller contains the following items:

Net Logon shares. These typically host logon scripts and policy objects for network client computers.
User logon scripts for domains where the administrator uses Active Directory Users and Computers.
Windows Group Policy.
File replication service (FRS) staging folder and files that must be available and synchronized between domain controllers.
File system junctions.

File system junctions are used extensively in the Sysvol structure and are a feature of NTFS file system 3.0. You must be aware of the existence of junction points and how they operate so that you can avoid data loss or corruption that may occur if you modify the Sysvol structure.