They refer to attitudes. Ethnocentrism is the view that one's culture's way of doing things is the right and natural way. All other ways are inferior, unnatural, perhaps even barbaric. Cultural relativism is the attitude that other ways of doing things are different but equally valid. It tries to understand the behavior in its cultural context.
Most humans are ethnocentric.
It is natural to think one's own culture's way is best. Anthropologists, however, strive to be culturally relativistic. If I were to tell most Americans about a group of people in Africa who sometimes kill healthy newborn infants, they would probably take the ethnocentric attitude that these people were barbarians. If I were to explain that they were hunters and gatherers living on the edge of starvation and that if a second child is born too close to the first, chances are about 100 percent that both will die because the mother does not have enough milk to support both, their attitude might change. They still might not condone infanticide, but they might understand it as the only viable choice in a desperate situation.