What is the difference between inner join and outer join?

An inner join returns only the rows that have matching keys in both tables, effectively performing a set intersection. An outer join (left, right, or full) returns all rows from one or both tables, filling in NULLs for missing matches. A left outer join keeps all rows from the left table, right outer join keeps all rows from the right table, and a full outer join keeps all rows from both tables. Outer joins are useful when you need to preserve data from one side even if there is no match, whereas inner joins are used when you only care about matched records.