Norman Personal Firewall has a very good list of default words to block, and will often stop a site that contains offensive content instantly. Sometimes this may not be enough, as a new site is launched NPF scans it, and based on the results of the scan NPF will attempt to stop the page loading.

NPF cannot block words that are embedded as pictures, and it may not necessarily stop all sites that are spawned from the original site, this is often seen as either pop ups or full pages. The developers of such sites are keen to attract people as they make money this way, and they use a variety of techniques to attract people, and stop content filtering work. Developers of e.g. personal firewalls are always one step behind them in that we have to learn the techniques they use, and then develop ways of defeating them.

Desktop content filtering within a home firewall is a relatively new concept, and is sure to take off over the next few years, however it will never stop access to all sites. This type of software is one of several tools that can be used as a first defence against children blundering into unacceptable web sites. However this is not a substitute for parental guidance prior to children using the Internet, and the browsing preferences of children can be audited periodically by checking the web access logs. You can then choose to block sites they are browsing to, or take alternative steps

The word list that NPF uses is not exhaustive, and some words are not included as they have multiple meanings. Such words can be included in the way explained above.