HTML Help:

This is currently the most popular electronic help format for Windows applications. It packs your entire help project into a single .CHM file. Fast, compact, excellent navigation and usability, universally compatible and full interaction with applications. Displayed by the HTML Help Viewer, that has been included with Windows since Windows 98.

Winhelp:

The predecessor of HTML Help. Consists of separate help (.HLP) and contents (.CNT) files. Still supported by all Windows platforms but navigation is less advanced and intuitive and appearance is not as slick as HTML Help. It is now  obsolete but full support is included for backward compatibility.

Browser-based Help:

An emulation of the HTML Help interface generated by Help & Manual and designed for use on the Web and intranets. Complete with a dynamic Table of Contents pane, keyword index and full-text search. Consists of a directory containing a large number of HTML files, graphics files and the files needed to display the Table of Contents etc.

eBooks:

This proprietary Help & Manual format packs your entire project into a single .EXE file with an integrated viewer program that can be displayed on any Windows computer, from Windows 95 to XP, without any additional software. This format also provides an emulation of the familiar HTML Help Viewer layout so that all users will be able to use it intuitively without additional explanation.

Adobe PDF:

Your project can also be output as a fully-formatted and full-featured PDF file that can be displayed on any computer with a PDF reader. Ideal for providing manuals that users can print themselves, either on CDs or for download.

Printed manuals:

The Print User Manual feature (in the File menu) generates a temporary PDF file in the background and outputs it to your printer. Also supports booklet format (multiple pages per sheet) and duplex printing.

MS Word RTF:

Provided for backward compatibility. Outputs your project to an MS Word Rich Text Format (.RTF) file.

Visual Studio Help:

This special format is also known as MS Help 2.0. It is provided for programmers who need to use MS Help 2.0 to document projects in Visual Studio .NET. It is not documented extensively in the help because Microsoft has not released it for general use under Windows. This format is irrelevant for normal application documentation.

For details see Visual Studio Help in the More Advanced Procedures section and the documentation of the MS Visual Studio .NET package. (If you don't have this package you don't need and cannot use Visual Studio Help.)

XML / XLIFF:

This format is used for translation of help projects into other languages. It exports your entire project to an XML file that can be edited by a translator who does not have Help & Manual. You can then re-import the translated XML document into Help & Manual, preserving all the formatting and data of the original project.

Longhorn Help:

Also known as Windows Vista Help. This format is not yet implemented because it has not yet been released. According to Microsoft, Longhorn Help will be the successor to HTML Help as the standard help format for Windows applications. Longhorn Help will probably be released with the new "Longhorn" version of Windows, which is currently scheduled to be launched at some time in 2006.

Visit the MSDN website for background information and the latest news on Longhorn Help.