UNIX : The difference between [ ... ] and [[ ... ]] (The extended test operator)

The [[...]] is processed as part of the shell grammar whereas [...] is processed like any other command.
Operators and operands are detected when the command is read, not after expansions are performed. 

1. The shell does not do word splitting or path-name generation inside [[...]].This allows patterns to be specified for 
   string matching purposes.
   
2. Parameter expansion and command substitution is allowed.
3. Prevents many logic errors in scripts. e.g [ "$x" = "TEST" ] can be written as [[ $x = "TEST" ]] the variables don't need to be quoted.
4. The &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.