A URL (Uniform Resource Locator) is a web address. Unlike a DOI, the URL of an item changes when the item's location on the Web changes. Because websites change frequently, URLs are not as stable as a DOIs.
Example;
http://www.tru.ca/library.html
If an online source has a DOI, then you do not need to include an URL (unless your instructor has expressly asked that you do so).
If an online source does NOT have a DOI, then you do need to include a URL. Which URL you use depends on the type of source, so use the tabs above to find out which URL to use.