Assuming you're joining on columns with no duplicates, which is by far the most common case:
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An inner join of A and B gives the result of A intersect B, i.e. the inner part of a venn diagram intersection.
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An outer join of A and B gives the results of A union B, i.e. the outer parts of a venn diagram union.
Examples
Suppose you have two Tables, with a single column each, and data as follows:
A B
- -
1 3
2 4
3 5
4 6
Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.
inner join
An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.
select * from a INNER JOIN b on a.a = b.b;
select a.*,b.* from a,b where a.a = b.b;
a | b
--+--
3 | 3
4 | 4
left outer join
A left outer join will give all rows in A, plus any common rows in B.
select * from a LEFT OUTER JOIN b on a.a = b.