The practice of yoga isn't just another form of exercise - another way of "working out." Yoga encompasses much more than just the physical poses we do in class. It also encompasses a system of beliefs - not religious beliefs - but beliefs about how we present ourselves and operate in the world - how we treat ourselves and how we treat others.
Karuna is the Sanskrit word for Compassion. In yogic philosophy, it is one of the Four Brahma Viharas, also known as the Four Divine Emotions, which are essentially principles for engaging in the world in ways that will result in contentment and happiness. The other principles are:
Metta: Loving kindness toward ourselves and others
Mudita: Sympathetic Joy (being happy for others)
Upekkha: Equanimity (acceptance and contentment)
A person who practices compassion - Karuna - is one who has an open and emphathetic heart that cares for everyone.