I believe that a left join gives you every record from the table before the join statement and all matching records from the records that match the criteria, substituting nulls where the table on the right or after the left join do not match. A right join works the opposite giving you all records from the table after the right join command and matching records from the table before the right join command and substituting nulls where there was no match yet still giving you the records from the table after the join. Does that make since.
Hope this helps
James