Yin/Yang: The term Yin/Yang is Chinese and refers to the active and passive principles of the universe. Yin refers to the female or [in-active] negative force; Yang to the male or active force. These two polar forces continually interplay with each other. The words are used to describe the constant “How” of motion and change in the Universe (i.e., the "Tao").
Yoga: The word Yoga comes from the Sanskrit word, “Yug” meaning Union with the Devine. Many yogic paths exist and include work on the physical body as in Hatha yoga, which uses breathing exercises and sustained physical postures to gain physical and mental control over the body; Bhakti, the yoga of devotion and love using the heart as a vehicle for transcendence; and other types of yoga specific to a path or teacher including Iyengar, Jnana, Karma, Kripalu, Kriya, Kundalini, Raja and Siddha yoga.
Yogi: A Yogi is someone who practices Yoga.