The 10 Heavenly Stems (天干) are pure, singular expressions of element + polarity: Jia (Yang Wood), Yi (Yin Wood), Bing (Yang Fire), and so on. They represent the visible, outward energy.
The 12 Earthly Branches (地支) are more complex — each is associated with an animal, a season, a time of day, and contains hidden Stems inside. They represent the environment, the hidden dynamics, and the body's roots.
Think of the Stems as the actors on stage — what you see. The Branches are the backstage — the sets, the hidden tensions, the real relationships. You need both to read a chart fully. Together they form the 60 possible pillar combinations (the sexagenary cycle).