In a literal sense, an online tour is a series of navigations between web pages with extra text, images and/or video which aren’t normally a part of the page added. The purpose of the extra content is to familiarise users to the workings of the interface of your website or web application.
The usual modus operandi of a tour is to run the first time a user signs up for a service. After this run through the tour does not run again unless the user specifically resets it. The extra content is usually added as a popup. Arrows are commonly used to point to the aspect of the interface being discussed. Next and previous buttons are often used to give the user control of the progress of the tour.
If your website is a simple brochure or informational site, then you don’t need a tour. If you’ve built a complex interface that users need to learn, it becomes far more useful. A tour can teach users much better than documentation can, as they are actually interacting with the user interface as they tour.
Online Tour Tools
We use Bootstrap Tour to build super awesome interactive tours. Bootstrap Tour is built on Twitter Bootstrap, a library of CSS and JS functions developed by Twitter designed to pull your site up by the bootstraps. Another tool for building tools is Tourmyapp. Many other tools are listed at this Quora question.