Think in Venn diagrams
INNER JOIN returns only matching rows. LEFT JOIN keeps all rows from the left table and matching rows from the right. RIGHT JOIN is the mirror of LEFT JOIN.
FULL OUTER JOIN returns matches plus unmatched rows from both sides (support varies by database).
Practice pattern
Write the same query four ways with different joins on two small tables of 5 rows each. Seeing the result set change is faster than memorizing definitions alone.
Sample query
Use this as a template while you practice.
SELECT u.name, o.total
FROM users u
LEFT JOIN orders o ON o.user_id = u.id;