[ in bash is a synomym for test but it's not still a builtin. For backward compatibility, it behaves similarly to /usr/bin/[ , which means variables among its arguments are still expanded by the shell, which is not true for [[ . In other words, you have to write things like

if [ "$x" = abc ]
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i.e. you need to quote the variables, while with `[[` you don't have to, as the shell processes the parameters before expansion:

if [[ $x = abc ]]
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Also, `[[` introduces pattern matching [[ $x = ab* ]] and `=~` you might now from Perl.