Winlogon is the component of Windows NT that provides interactive logon support. Some aspects of Winlogon are replaceable. In particular, the identification and authentication aspects of Winlogon are implemented in a replaceable dynamic-link library (DLL). This replaceable DLL is referred to as the Graphical Identification and Authentication DLL, or GINA. GINA allows developers to implement smart-card, retinal-scan, or other authentication mechanisms in place of the standard Windows NT user name and password authentication.
A sample GINA.DLL and online documentation are available in the Windows NT Software Development Kit (SDK).