URIs and URLs are both ways to locate information. URI is a Universal Resource Identifier, and URL is a Universal Resource Locatior. All URLs are URIs but not all URIs are necessarily URLs. There is one more lesser known group called URN or Universal Resource Name. A URI can be a name (RUN), locator (URL) or a combination of both, but a URL is always just a locator, and a URN is always just a name. A URL includes a locator as well as the protocol used to access it, generally HTTP or HTTPS. An example of a URN is an ISBN number, used to identify a specific book. 

In SEO, we use URLs to help give the search engines semantic understanding through the site architecture. There is also some evidence that including keywords in URLs can help drive relevance and rankings.