From Paterva Wiki
(Uniform Resource Locator) http://www.abc.com/main.html is a URL. http://www.abc.co.uk/news.asp?id=1 is also a URL. Even ftp://ftp.abc.sub.co.za/ is a URL. In Maltego URLs are (usually) collected when executing search engine transforms on entities such as phrases, persons etc. The URLs are collected in the output entities (websites, email addresses etc).
Learn more about this entity
URL on Wikipedia
See also (other entities)
Properties of this entity in Maltego
What is mildly interesting is that Maltego does not keep URL as the main value in this entity (because rendering a URL is just messy). Where possible the URL's truncated title is kept in the value. If the title is not available, the truncated URL is stored in the main value. The actual URL is kept in the (Actual URL) field. Additionally the full title is kept in the (Full Title) field.