An inner join retrieves only the records that have corresponding values in both tables, while an outer join returns all records from one table along with the matching records from the other table.

In SQL, a join operation combines rows from two or more tables based on a related column between them. The two most prevalent types of joins are the inner join and the outer join. An inner join specifically returns only those rows where there is a match in both tables. If a row in the first table corresponds to a row in the second table, that row will be included in the result set. Conversely, if there is no match, the row will not appear in the result. Essentially, an inner join returns the intersection of the two tables.

In contrast, an outer join returns all rows from one table along with the matched rows from the other table. If there is no corresponding match, the result will show NULL for the columns of the table without a match. There are three types of outer joins:

1. Left Outer Join: This returns all rows from the left table and the matching rows from the right table.
2. Right Outer Join: This returns all rows from the right table and the matching rows from the left table.
3. Full Outer Join: This returns all rows that have matches in either of the two tables.

The fundamental difference between an inner join and an outer join lies in the result sets they produce. An inner join will return only those rows with matches in both tables, whereas an outer join will return all rows from one table and any corresponding rows from the other table. Consequently, an outer join may yield more rows than an inner join. The decision to use an inner join or an outer join in SQL depends on the specific needs of your query.