Cisco IOS release 12.4(20)T introduced (web) content filtering. The feature is aimed primarily at the ISR routers which can connect to an external service hosted by Trend Micro, turning the mundane administrative task of blacklisting web sites into a subscription service.
You can use the same features without the Trend Micro service whenever you need a quick fix (for example, blocking a few web sites). The Local content filtering is also available on other platforms, not just on ISRs. Not surprisingly, the online IOS documentation on the local part of this feature is a bit sketchy, but you’ll find all the details you need to configure this feature in the Local Content Filtering article.