|Specifying an url related to a feature.|
|Used on these elements|
|Status: in use|
|Tools for this tag|
URL means Uniform Resource Locator. It can stand for a variety of things while mappers mostly use it for websites.
This tag is being used for very different types of content - official websites of an amenity or its operator, third-party pages, the object's Wikipedia entry, photographs featuring the object, or even documentation on the OSM wiki. So it is advised not to use this tag when more specific alternatives are available such as website=*, contact:website=*, facebook=* (or its synonym contact:facebook=*), twitter=* (or its synonym contact:twitter=*), image=*, wikipedia=* and so on.
Creating a robust tag
To create a tag that will stand the test of time, simplify the URL as much as possible. For example http://www.example.com/department/index.shtml?foo=fum is more likely to work in the future if coded as http://www.example.com/department/ or better yet http://www.example.com