Quoted characters are expanded by own rules. A character following the escape character "\" is taken as literal. "\$" is taken as literal "$" instead of the beginning the start of a variable expansion:
$ echo \$USER $USER
Only the sequence "\newline" has a special meaning. \newline is interpreted as line continuation:
$ echo 12\ 34 1234
Single quoted strings are taken always literal:
$ echo '$USER' $USER
$ echo "$(echo $USER)" pa
"\" disables the special meaning of characters within double quotes:
$ echo "\"Oranges and lemons\"" "Oranges and lemons"
To translate special characters, ANSI-C Quoting can be used. Escape sequences between "$'...'" operator:
echo $'\n\t\\$USER' \$USER
will be translated according to the ANSI-C standard: \n - newline, \t - tab, \\ backslash.