Context-sensitive help is a kind of online help

Online help

Online help is topic-oriented, procedural or reference information delivered through computer software. It is a form of user assistance. Most online help is designed to give assistance in the use of a software application or operating system, but can also be used to present information on a broad...

 that is obtained from a specific point in the state of the software, providing help for the situation that is associated with that state.

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.

Context-sensitive help can be implemented using tooltip

Tooltip

The tooltip or infotip is a common graphical user interface element. It is used in conjunction with a cursor, usually a mouse pointer. The user hovers the cursor over an item, without clicking it, and a tooltip may appear—a small "hover box" with information about the item being hovered...

s, which either provide a terse description of a GUI widget or display a complete topic from the help file. Other commonly used ways to access context-sensitive help start by clicking a button. One way uses a per widget button that displays the help immediately. Another way changes the mouse pointer shape to a question mark, and then, after the user clicks a widget, the help appears.

Context-sensitive help is most used in, but is not limited to, GUI

Gui

Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...

 environments. Examples are Microsoft's

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 WinHelp, Sun's

Sun Microsystems

Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

 JavaHelp

JavaHelp

JavaHelp refers to both an application and a format for online help files that can be displayed by the JavaHelp browser. It is written in Java, and is mainly used in Java applications...

, OS/2

OS/2

OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

's INF Help

Information Presentation Facility

Information Presentation Facility is a system for presenting online help and hypertext on IBM OS/2 systems. IPF also refers to the markup language that is used to create IPF content. The IPF language has its origins in BookMaster and Generalized Markup Language developed by IBM...

 or Panviva's SupportPoint.
A similar topic is embedded help, which can be thought of as a "deeper" context-sensitive help. It generally goes beyond basic explanations or manual clicks by either detecting a user's need for help or offering a guided explanation in situ. Embedded help is not to be confused with a software wizard.