Regular Expression differences between [] and ()

How to understand about this Pattern?

/^((?:\+?61)|0)(2((3[3789])|(4[0123456789])|(5[012356789])|
(6[0123456789])|[789][0-9])|(3((3[234])|(4[012345789])|(5[0123456789])|
(6[123457])|[789][0-9]))|(7(([23][0-9]|(4[0123456789]))|(5[234567])|
(7[679])))|(8((2[56])|(5[123458])|(6[0123456789])|(7[0123456789])|
(8[0123456789])|(9[0123456789]))))[0-9]{6}$/

How the square bracket[] and Parenthesis() differ from?