See the following web page for a complete explanation and definition of the different join types:
Left_join#Left_outer_join
when you create a join the comparison has a table on the left and a table on the right:
INNER JOIN LeftTBL.IdColumn ON RightTBL.IDColumn
Whether to use a Right or left join depends on the table on the right or left, the data in each of those tables and the expected results.