Life release, or "tsethar" in Tibetan, is a Buddhist practice of saving lives within both Theravada and Mahayana Buddhist tradition. By buying animals who are destined to be killed and releasing them in their native environments, life release puts compassion into action. Unfortunately, most of the time we hardly realize that as human beings we have this precious opportunity.

Although every life is precious, the process of living inevitably causes us to take the lives of other living beings. We cannot completely prevent this situation because as long as we walk, breathe, eat, and so forth, we cause the deaths of many creatures. However we can cultivate mindfulness, and try to reduce taking lives to the best of our ability. We can also offer a gift of life and protection through the practice of Life Release. No matter what our lifestyle, we can do this practice. It benefits those who offer the gift of life as well as those who receive it. And regardless of the religion we practice, its results will be strengthened if the practice is conducted with an aspiration that all beings without exception enjoy happiness and a life free from any harm.
In Buddhism the practice has two aspects: 1. The act of saving lives, 2. The blessing of the prayers that accompany life release. Specific practice instructions can be found at Kagyu.org, source of the above text. For more information on tsethar, please see also the page on Chatral Rinpoche.