The Yogacara School asserts that everyone has a mental continuum from beginningless time until the attainment of enlightenment. This mental continuum is sometimes comprised of the five consciousnesses of the eyes, ears, nose, tongue and body, but sometimes not. No matter how it manifests itself, the permanent existence of what is called the alaya consciousness is present at all times. Whenever actions are committed, a karmic seed will be planted in the alaya consciousness.
There is an analogy for this. When ink is poured on snow, the snow will take on the color of the ink. After the snow melts, the ink’s color can still be seen on the ground. Similarly, if karma is born of defilements, a karmic mark will be left in the alaya consciousness after the defilements are gone.
~ Luminous Wisdom 1 : On Cause and Effect