2006 - Map Features Tag Documentation Added

The genius of OpenStreetMap lies in the simplicity of its model, making it easy for anyone to grasp how it works, and the versatility of its tagging system, which has near-infinite capacity to describe the physical world. 

The conceptual data model is based on three types of elements: nodes (defining points in space); ways (defining linear features and area boundaries), and relations (defining how other elements work together). These elements derive their meaning from tags which are comprised of a key and a value. The key describes a topic, category, or type of feature (e.g., highway or name). The value provides detail for the key-specified feature. Commonly, values are free form text (e.g., name=”Green Arbor Memorial Highway”) (source).

Early on, developers had to work out a way to protect the necessarily unrestricted nature of the keys and values, while also describing a core set of map features. You can still see the wiki where the documentation for this base map was collectively created. 

As of February 2024, OpenStreetMap contained over 96 thousand distinct keys and over 155 million distinct tags.