URL stands for Uniform Resource Locator. It is a Web site address. It is what lets you locate a particular site. For example, wiki.answers.com is the URL for the WikiAnswers home page.
The domain name is the first part of a URL. Private, corporate, or commercial domains usually end in .com. Government domains usually end in .gov. Non-profit organizations and trade associations usually end in .org. Other countries use different endings, or "top-level domains."