IE uses the settings from CP->Internet Options->Content Tab to
filter this stuff, assuming the sites have volunteered to declare the content type.
It actually works better than 60% of the time, but no filter will block 100%.
Sadly, Firefox and other browsers do not reference these Internet Options.
Then there's the keyword filtering provided in some routers; add keywords like
pron, xxx (you make get false positives with this one), many of the 4-letter
smut words (F*** BLO**** P***Y ...) and you will block another large percentage.