A 'Union selects rows' and Join 'selects columns from 2 or more tables'.

JOIN: A Join is used for displaying columns with the same or different names from different tables. The output displayed will have all the columns shown individually. i.e. The columns will be aligned next to each other. UNION: The UNION set operator is used for combining data from two tables which have columns with the same datatype. When a UNION is performed the data from both tables will be collected in a single column having the same datatype.

A join selects columns from 2 or more tables. A union selects rows

joins is used in our query
like,
SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2
ON table_name1.column_name=table_name2.column_name

and union is used to merge the result of two query

like,

select * from table1
union
select * from table2