Start situation (download the sql script here)

Table A Table B
1 2
2 3
3 4

The INNER join (“normal / regular” join)

select a.id as A, b.id as B from a inner join b on a.id = b.id; 

Results
A B
2 2
3 3

Only rows that have the same id in both tables appear in the result set.

The LEFT OUTER join

select a.id as A, b.id as B from a left join b on a.id = b.id; 

Results
A B
1 NULL
2 2
3 3

Every row from the table mentioned on the left (table A in this example) will be part of the result set. If possible the corresponding row (the one with the same id) in table B is also part of the result set. If no corresponding row is found in table B NULL values are provided.
The RIGHT OUTER join

select a.id as A, b.id as B from a right join b on a.id = b.id; 

Results
A B
2 2
3 3
NULL 4

Every row from the table mentioned on the right (table B in this example) will be part of the result set. If possible the corresponding row (the one with the same id) in table A is also part of the result set. If no corresponding row is found in table A NULL values are provided.
The FULL OUTER join

select a.id as A, b.id as B from a full join b on a.id = b.id; 

Results
A B
1 NULL
2 2
3 3
NULL 4

Every row from table A and table B is part of the result set. If possible a match is made based on the id. If a match could not be made NULL values are provided for the missing items.
Cartesian product
select a.id as A, b.id as B from a , b;
Results
A B
1 2
2 2
3 2
1 3
2 3
3 3
1 4
2 4
3 4

Every row in table A is joined with every row in table B. So the cartesian product of table A and table B delivers 9 rows in the result set. A cartesian produkt is often specified by mistake (missing join condition).