A natural join is a join that creates an implicit join based on the same column names in the joined tables.
The following shows the syntax of the PostgreSQL natural join:
```
SELECT select_list
FROM T1
NATURAL [INNER, LEFT, RIGHT] JOIN T2;
```
A natural join can be an inner join, left join, or right join. If you do not specify a join explicitly e.g., INNER JOIN, LEFT JOIN, RIGHT JOIN, PostgreSQL will use the INNER JOIN by default.
If you use the asterisk (*) in the select list, the result will contain the following columns:
* All the common columns, which are the columns from both tables that have the same name.
* Every column from both tables, which is not a common column.
```
SELECT * FROM products
NATURAL JOIN categories;
```