You can go one of two ways as a tour bus driver, sticking strictly to driving, or adding the role of commentator to your tasks, which makes you a driver-guide if you do. In either case you need a Commercial Driver's License (CDL), and when on duty, must update your driving log constantly. Department of Transportation (DOT) personnel regularly stop bus drivers and ask for their logs, and not having one to show creates an instant problem, usually involving the bus not moving for 4 or 6 or 8 or more hours as a penalty. That has a huge and not so good impact on the tour. Most countries have similar licensing requirements but with different names.
A bus driver on most over-the-road trips works with a tour director to provide the best possible experience for the guests. The TD is in charge of the tour, is responsible for the route and the commentary, and directs the driver as to where the group will go. Good drivers add a lot to guests' enjoyment of the tour. Passengers want to see that the driver and the TD work well together; tension between them is readily apparent and negatively noted. The driver is responsible for getting luggage on and off the bus, and often offers bottle of water for sale to the guests (not the TD, whose water is always free). The TD provides commentary on the bus, and the bus driver generally has no need to be on the microphone, though some do use it, not always with the TDs approval. Drivers work for bus companies that contract with tour companies to provide over-the-road buses.
On some over-the-road tours, and most in-town sightseeing tours, the driver also provides commentary, and is called a driver-guide. It is no small task to drive passengers safely around, say, Chicago, while pointing out landmarks and describing the history of specific locations. Some of the largest tour companies providing tours to and from Alaska, also create week-long land tours for their guests, using driver-guides. On those tours, tour directors are there to organize the details of the trips, yet it is the driver who is on the microphone talking. Driver guides work for sightseeing companies in a particular town, and generally drive only in a local area and not on long over-the-road tours. The exception to that is the tour leader on an adventure tour, as the group moves from one adventure location to another. Such tours generally use 15 or less passenger vans rather than buses, and the driver provides commentary while driving, usually without benefit of a microphone.