An Outer Join is a type of database operation that combines rows from two or more tables based on a related column. Unlike an inner join, which only returns matching rows, an outer join includes all rows from one table and the matched rows from the other. If there is no match, the result will still include the row from the first table, but with null values for the columns from the second table. There are three main types of outer joins: Left Outer Join, Right Outer Join, and Full Outer Join. A left outer join returns all rows from the left table and matched rows from the right table, while a right outer join does the opposite. A full outer join combines the results of both left and right outer joins, including all rows from both tables, with nulls where there are no matches.