Destination URLs are examined for compliance with corporate policy and for malicious potential of destination Web sites. URL filtering identifies certain types of Web sites, such as known malicious sites and sites that display inappropriate or pornographic materials, and allows or blocks access to these sites, based on corresponding, predefined URL categories. You can allow or block access to configured categories on selected days of the week (for example, over the weekend), or at specific times of the day (for example, during work hours). URL filtering checks the URLs against Microsoft Reputation Services, which combines multiple sources to increase the coverage of URLs and categorization and features over 90 URL categories (such as, Malicious, Anonymizers, or Illegal Drugs). According to the policy created by the Forefront Threat Management Gateway administrator, URL filtering either allows or blocks access to the requested site. More information here.