These tools use a list of keywords found on Web sites deemed inappropriate, such as hateful racial terms on hate sites or words frequently used on sexually explicit sites. Some keyword-based filters are designed very carefully, so that when the program encounters a word that might have some inappropriate connotations, like "breast," it doesn't filter the site without first looking at the context that surrounds it. Analyzing context helps to avoid slipups -- it can prevent "breast cancer" from being filtered out.
These tools are generally less precise than human-maintained filters. The kinds of mistakes most frequently made by these filters include blocking both sexually explicit/adult material as well as sites against such material or discussing censorship of it. This is because the same groups of words sometimes appear together in all three contexts.
Parents can wield a good deal of control over some keyword-based filters. They can edit the list of keywords, or specify that certain sites they've examined on their own should be exempt from the program's filtering rules. On the other hand, some of the complicated rules for analyzing the context in which keywords appear can be difficult or impossible to edit. These rules are carefully constructed, and use elaborate computational pathways to arrive at the most accurate results possible, but as a result, they can't usually be altered by a user who finds them too restrictive.
Keyword-based filters are generally the most scalable of the Web filters, since they don't rely on predetermined lists to filter. When a keyword-based filter encounters a site it's never seen before, it analyzes the content right away, comparing it to the rules with which it's been programmed to arrive at a rapid decision, without having to wait for its lists to be updated or for people to examine and rate the page.
Physical safety protections
Keyword-based filters can do a good job at controlling the access kids have to inappropriate Web pages. However, many do not apply the same kinds of analysis to other forms of communication, like email or chat rooms -- the forms of communication most likely to lead to a dangerous face-to-face meeting.
Because these tools analyze and filter while the user is on the computer, they may slow down browsing online. Also, they may inappropriately limit access to online information that is appropriate for kids.