Wikipedia
Co-browsing, in the context of web browsing, is the joint navigation through the World Wide Web by two or more people accessing the same web pages at the same time.
Early co-browsing was achieved by local execution of software that had to be installed on the computer of each participant. More advanced tools didn't have to be installed, but still required local execution of software or at least web-browser plug-ins, extensions, or applets. Most tools were limited to a single user that was able to navigate, while the others could only watch.
Some tools provide very limited co-browsing by only synchronizing the page location (URL) of the page that should be shared. Full co-browsing supports automatic synchronization of the browsers' state and content, including frames, portlets, or even content of the form fields and controls. Some tools can even identify complex media objects such as audio and video players and offer capability of synchronous (coordinated) playback with start/pause/stop functionality.
Co-browsing is difficult to implement due to the essential confidence requirements to share any real-time experience, and strong resistance provided by OS and browser security mechanisms. Co-browsing technology has many inherent challenges such as page personalizations or sites that require user authentication.