SENS is an acronym for the System Event Notification Service.

On Windows XP/W2k3 SENS is baked into Winlogon - in Vista+ it is a separate service hosted by one of the svchost.exe instances on the system.

SENS is purely informational and reactive - other components subscribe to SENS notifications or send notifications about their status to SENS. 
Those components may in turn do something as a result of getting a SENS notification - the role of SENS is only to send and receive notifications.

Example: After the screen saver on the system is started it posts a StartScreenSaver event to SENS to indicate it has started, any other services that have subscribed to SENS notifications get notified that this SENS event has occurred.


 

System Event Notification Service
http://msdn.microsoft.com/en-us/library/windows/desktop/cc185680(v=vs.85).aspx

SAMPLE: Using ISensNetwork (SensNetDemo.exe)
http://support.microsoft.com/kb/321381

You receive error 0x80240020 when you use Windows Update Web site, Microsoft Update Web site, or WSUS to install updates
http://support.microsoft.com/kb/910341