What is difference between URL and URI in Salesforce?

URI: Uniform Resource Identifier (URI) is a string of characters used to identify a name or a resource on the Internet. Such identification enables interaction with representations of the resource over a network (typically the World Wide Web) using specific protocols
URL: In computing, a Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it.
Example
To identify a specific resource and how to access it - in all completeness
URI: mysql://localhost@databasename:password
The URL shows you where you can find the database on the internet and which protocol you should use.
URL: mysql://localhost

HII Aditya,
URL is a subset of URI that specifies where a resource exists and the mechanism for retrieving it, while URI is a superset of URL that identifies a resource. The main aim of URL is to get the location or address of a resource whereas the main aim of URI is to find a resource.