In Microsoft HTML Help, the Contents tab is on the left side of the HTML Help window. Not only is the Contents tab always available, but it can "track" where the user is in the Help system. Whenever the Contents tab is selected, it shows the location in the table of contents of the currently selected topic. (The topic text appears on the right side of the window.)
The instructions for the organization and appearance of the Contents tab are part of the .HHC file. HTML tables of contents use the book and pages metaphor, just like WinHelp Help systems. But unlike WinHelp, you can replace the book and page icons with other images of your choice to customize the Contents tab.