In many cities, guided city tours are organized by professionals, companies and local organizations for those wanting to explore the city with professional guidance.
There are many different types of tours throughout the world. Tours can be valuable in destinations that are particularly difficult to navigate (like Fez) or where a lot of interesting information is not obvious to a casual visitor (like Pompeii). When deciding on a tour, think about the layout destination—a walking tour might be better in an old, compact city with many pedestrian-only streets, whereas a boat tour might be the way to go in a city of canals. Hotels and hostels often have information about available tours.
If you are going to spend more than a day at a destination, a tour is usually a good starter activity, for several reasons. It provides a sense of orientation, an overview of attractions to see later, and a chance to find travel companions (especially when travelling alone).
Large cities and well-visited destinations usually have many competing tour services. They might have tours to different districts, on specific themes.
The most straightforward type of guided tour is a walking tour, where a guide leads their patrons through the city on foot. As there are no significant costs incurred but the time of the guide, there are many cities where people volunteer to guide tourists around for free, e.g. as part of the Global Greeter Network. In countries with tipping cultures, guides should be tipped—especially when the tour is free!
Some tour operators use a free tour as a sales pitch for paid tours and events, often providing discounts.
Larger cities can also be toured by bus or coach, usually involving stops for walking tours of selected areas. A more recent development that gained much popularity is the hop-on / hop-off bus tour, where buses run along a fixed route and stop at fixed points at given hours, and patrons can choose to get on or get off the bus at any stop at their whim, at a fixed price.
Another option is to make use of local public buses - circle lines are particularly good for that - which "just so happen" to stop at local landmarks. While you won't get a running commentary and the buses won't be open top, the price for a tour will be much lower.
Some cities with extensive above-ground urban rail systems offer city tours, sometimes in historic rolling stock. Those tours tend to be available for special events only and the vehicle is often as much part of the attraction as the tour. One obvious downside is the fact that the tour is limited to where tracks go.
For those who don't like walking, in many cities tours are organized using Segways, the two-wheeled single-person electric mobility devices.
Popular in canal cities and in areas with important rivers and lakes. They vary from chartered boat tours where you're just in a small group to larger groups with 20 people or so. An expanded version of boat tours are cruises, sometimes with thousands of people on one boat.