An inner (standard) join returns a row in the result, when the join finds a match between two tables, and no results when there is not match.
A left join always returns at least a row; when there is no match, the part of the result (if selected) that corresponds to the right table, has NULLs.
There are many important consequences of this, one of which is that left joins enforce table ordering; inner joins can be reordered by the optimizer if iterating them in a reordered way gives faster access (imagine two nested for loops, and swap the inner loop with the outer one).
A left join always returns at least a row; when there is no match, the part of the result (if selected) that corresponds to the right table, has NULLs.
There are many important consequences of this, one of which is that left joins enforce table ordering; inner joins can be reordered by the optimizer if iterating them in a reordered way gives faster access (imagine two nested for loops, and swap the inner loop with the outer one).