APA Style Guide

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. If an online work has both a DOI and a URL, include only the DOI.

https://doi.org/xxxxx

• Present both DOIs and URLs as hyperlinks (i.e., beginning with “http:” or “https:”).
• It is acceptable to use either the default display settings for hyperlinks in your word-processing program (e.g., usually blue font, underlined) or plain text that is not underlined.
• Leave links live if the work is to be published or read online.
• The string “https://doi.org/” is a way of presenting a DOI as a link, and “xxxxx” refers to the DOI number.
• The preferred format of the DOI has changed over time. Although older works use previous formats (e.g., “http:/dx.doi.org/” or “doi:” or “DOI:” before the DOI number), in your reference list, standardize DOIs into the current preferred format for all entries. 
• Copy and paste the DOI or URL from your web browser directly into your reference list to avoid transcription errors. Do not change the capitalization or punctuation of the DOI or URL.
• Do not add a period after the DOI or URL because this may interfere with link functionality.