Pretty simple actually.

Distinct column1 will give the distinct values within the column1

Distinct * will give the distinct rows within the table.

Say this is the content of table1

Column1 Column2

1001 A

1001 B

1001 C

1002 A

1002 B

1003 C

1003 C

Select Distinct column1 from table1 will give following result:

Column1

1001

1002

1003

Distinct * from table1 will give following result:

Column1 Column2

1001 A

1001 B

1001 C

1002 A

1002 B

1003 C

As only <1003>,<C> is the row (*) which is duplicate.