练习 (liànxí) is both a verb meaning 'to practice' and a noun meaning 'exercise, practice.' It's the general word for practicing skills like languages or instruments. In contrast, 练 (liàn) is a verb-only shorthand often used in set phrases (e.g., 练字), while 锻炼 (duànliàn) specifically means physical exercise or purposeful training (e.g., 锻炼身体).