squidGuard can be used to
- limit the web access for some users to a list of accepted/well known
web servers and/or URLs only.
- block access to some listed or blacklisted web servers and/or URLs
for some users.
- block access to URLs matching a list of regular expressions or words
for some users.
- enforce the use of domainnames/prohibit the use of IP address in
URLs.
- redirect blocked URLs to an "intelligent" CGI based info page.
- redirect unregistered user to a registration form.
- redirect popular downloads like Netscape, MSIE etc. to local copies.
- redirect banners to an empty GIF.
- have different access rules based on time of day, day of the week,
date etc.
- have different rules for different user groups.
- and much more..

Neither squidGuard nor Squid can be used to
- filter/censor/edit text inside documents
- filter/censor/edit embeded scripting languages like JavaScript or
VBscript inside HTML