Realism is the belief that everything is clearly defined and can be measured. For example, realists believe that a desk is a desk, an hour is 60 minutes long, and in the case of treating Depression, they state that by asking the patient a few questions they can diagnose how severe their depression is and plan a course of treatment to cure it.
Relativists on the other hand believe that everything is relative, that nothing can be clearly defined or measured as everything is viewed differently by different people. For example, a desk may be a desk to one person but to another person who may be sitting on it, it could be a seat. An hour may be 60 minutes long but to some people it may feel shorter and to others it may feel longer. And in the case of treating Depression, they believe that each person's experiences of it is different and therefore asking questions to determine its severity is pointless as some people may feel their depression is severe when it's technically not and others may feel their depression is mild when technically it's not.
Realists are therefore more into scientific methods and finding proof. They like gathering data and statistics to figure out how many people suffer from a certain condition such as Depression. Whereas Relativists are more into figuring out how certain conditions affect people and how they deal with things rather than worrying about statistics.