^^ Thats the "faster" bit. OpenDNS also does content filtering, but for that you need to set up an account (still free) and enable what you want filtered in your profile, which is the "safer" bit.
For example, suppose you are redirected to a malicious site, say
www.somebadsite.com. Normally your browser sends a dns request which gets resolved to an IP address and off you go to that site. I presume what OpenDNS does is deliberately block entries for known bad sites (that could be porn, malware, ad servers, whatever etc) by pointing them to an invalid IP address such as 127.0.0.1 (localhost). So if you try to visit or are directed to a malicious site, it's just silently blocked - you would just see a standard "page not available" error just as if the site didn't exist. By default, nothing is blocked - I think you select the categories you'd like to have blocked and it's stored in your profile. This is actually an incredibly efficient way of blocking sites and far more effective than using an add on application to do it.