This is in the VFP 8.0 help under select - sql. I thought is explained inner/outer join conditions pretty well. I hope this helps

Because SQL is based on mathematical set theory, each table can be represented as a circle. The ON clause that specifies the join conditions determines the point of overlap, which represents the set of rows that match. For an inner join, the overlap occurs within the interior or "inner" portion of the two circles. An outer join includes not only those matched rows found in the inner cross section of the tables, but also the rows in the outer part of the circle to the left, or right, of the intersection.