I don't know that this question actually requires a historian to answer but....

Battle means an armed confrontation with only limited or improvised fortifications, shorter duration than a war.

Siege means an armed confrontation between two forces involving extensive fortifications.

Sack means an attack by an armed group against an undefended or no longer defended city or other valuable location. The term implies looting and possibly atrocities by the armed group, either deliberately or as a lapse in discipline. A sack may occur after a Siege or Battle.