In SQL, an outer join selects all of the lines (tuples) in one table with the matching lines in the other. There are two types of outer joins, the left outer join and right outer join. In a left outer join you select all of the lines from the first table, in a right outer join you select all of the lines from the second table.
The syntax is usually something like:
SELECT * FROM a LEFT JOIN b ON a.field=b.field
Compare with join (cross join) and inner join.