Uniform Resource Identifier (URI)
A unique sequence of characters that identifies a logical or physical resource used by web technologies, such as a standard web address (http://wisc.edu). [Wikipedia link]
Uniform Resource Locator (URL)
A unique sequence of characters that identifies a logical or physical resource used by web technologies, such as a standard web address (http://wisc.edu). Although URL is a specific type of Uniform Resource Identifier (URI), the terms are often used interchangeably. [Wikipedia link]
User Interface (UI)
The space where interactions between humans and machines occur. [Wikipedia link]