URL/Content Filtering

Content filtering is the technique whereby content is blocked or allowed based on analysis of its content, rather than its source or other criteria. It is most widely used on the internet to filter email and web access.
Content filtering is the most commonly used group of methods to filter spam. Content filters act either on the content, the information contained in the mail body, or on the mail headers (like "Subject:") to either classify, accept or reject a message.