Originally, the character for yang depicted rays coming down from the sun, combined with 'hill', meaning the sunny side of a hill (the development and transition of the character depicted above, from original to the modern, left to right).
Yang is inseparable from yin. They describe the relative, mutually dependent natures of any given event or entity. Yang represents that aspect which is more active and evident. It is, for example, the execution of a plan. It is the unfolding or expressing of inner potential and ability.
Yang is more external, or evident, than yin. For instance, in Chinese medical anatomy, the skin is more yang than the muscles. (However, the muscles are more yang than the bones.) The two cycle and transition from one to the other. Activity is defined by its motion compared to lack of motion, but it will inevitably transform back to stillness.
Yin and yang find their birth within the other. Summer is the most yang season, but that which makes it so - the flowers in full blossom, the vigor and vibrancy in the air - is rooted in, was born within winter, and within that summer, the seeds of winter are born...