Context-sensitive help is a form of help dialogue or information that is specificly designed for the current state of an application interface. Context-sensitive help is often embedded or accessible via the gui in form of tooltips, popups or buttons that trigger the opening of the help. For these reasons, context-sensitive help is often short and specific.
Context-sensitive help, as opposed to general online help or online manuals, doesn't need to be accessible for reading as a whole. Each topic is supposed to describe extensively one state, situation, or feature of the software.
(retrieved from http://en.wikipedia.org/wiki/Context-sensitive_help - 2013-02-05)