UNION joins two results and remove duplicates, while UNION ALL does not remove duplicates. UNION also sort the final output.
UNION
UNION ALL
W
1,1: select 1 from dual union all select 1 from dual 1: select 1 from dual union select 1 from dual
select 1 from dual union all select 1 from dual
select 1 from dual union select 1 from dual