Cellars are very common, most houses have one. But there are differences. Some cellars from the Middle Ages are really huge and have an interesting story. That is the sort of cellars which is listed on showcaves.com if it is open to the public. We will not list your basement, don't panic.
Some cities are built on soft rocks, like sandstone, tufa, or travertine. Those rocks are so soft, sometimes it is even possible to dig a hole using a pickaxe and a shovel. Travertine is cut with a saw, as long as it's still underground and humid.
Often, there is a second benefit in building a huge cellar: the removed material may be used for another purpose, building houses, sand for mortar or for cleaning (as an abrasive). So many cellars are actually former quarries which were reused.
Cellars were used to store all goods which must be kept cool to stay fresh. This works with most food, especially vegetables, potatoes, roots, and more. Also, it is possible to store many different things together. There is normally no need to keep them separated. Nevertheless, there are some specialized cellar types, which were built and used for just one purpose.
Then there are the so-called Kellergasse (cellar street), which are widespread especially in wine-growing areas. Sometimes the term Kellertrift or cellar quarter is also used. Cellars are built in rows along a street or a narrow path. This was mainly done when it was not possible to build cellars in the village, for example because of a high groundwater level. In rare cases, cellars are not built underground at all, but above ground. After the vault has been built, it is covered with metres of earth or rock. A great advantage is the level access, often wide enough that the cellar can be accessed by vehicles.