What is the difference between a website and an URL?

A URL is THE address to a website.

http://www.answers.com/topic/url

URL is an abbreviation of Uniform Resource Locator, the global address of documents and other resources on the World Wide Web.

A web site (location) on the World Wide Web. Each Web site contains a home page, which is the first document users see when they enter the site. The site might also contain additional documents and files. Each site is owned and managed by an individual, company or organization.

http://www.webopedia.com/

Other things besides websites can have a URL. For example, a URL starting with ftp:// points not to a website but to a file download site.