What is a trigger in SQL?

CREATE TRIGGER validate_age BEFORE INSERT ON students FOR EACH ROW BEGIN IF NEW.age < 18 THEN SIGNAL SQLSTATE '45000'; END IF; END;

When does this trigger fire?

CREATE TRIGGER after_emp_insert AFTER INSERT ON employees FOR EACH ROW INSERT INTO audit VALUES (NEW.id, 'INSERT', NOW());

What does this trigger do?

CREATE TRIGGER update_timestamp BEFORE UPDATE ON employees FOR EACH ROW SET NEW.last_modified = NOW();

What is NEW referring to?

How do you remove a trigger?