BDSM can be practised in a zillion different ways because there are as many people who do. In the interest of sexual gratification and pleasure, some people engage in aspects of BDSM only while having sex or on rare occasions. A full-time way of life for some people involves BDSM; they have roles established in every aspect of daily life and social interactions.
BDSM frequently involves some sort of power dynamic or imbalance, but pain is not always a part of it. As an illustration, one partner may perform sexual favours for the other during BDSM play.