URL stands for Uniform Resource Locator. A URL is nothing more than the address of a given unique resource on the Web. In theory, each valid URL points to a unique resource. Such resources can be an HTML page, a CSS document, an image, etc. In practice, there are some exceptions, the most common being a URL pointing to a resource that no longer exists or that has moved. As the resource represented by the URL and the URL itself are handled by the Web server, it is up to the owner of the web server to carefully manage that resource and its associated URL.
DOI stands for a Digital Object Identifier. A DOI is a persistent identifier or handle used to uniquely identify various objects, standardized by the International Organization for Standardization (ISO). They are widely used to identify academic, professional, and government information, such as journal articles, research reports, data sets, and official publications. DOIs have also been used to identify other types of information resources, such as commercial videos. The DOI for a document remains fixed over the lifetime of the document, whereas its location and other metadata may change.
Referring to an online document by its DOI should provide a more stable link than directly using its URL. A DOI is preferable, but if a URL is the only location available, provide it instead.