Just like a person, a family comes during its lifecycle through a series of so called normal crisises, overcoming them with more or less ease.
According to modern psychological theories, a family is a system, where whatever happens to it's members, affects all others and the state of family as a whole.
Signs of family problems often manifest themselves in unusual changes of kids behaviour. For instance, a school kid starts to piss into his bed (enuresis).
Family therapy sessions could conducted individualy, in pairs, or include all family memebers.
Usually, the aim is to work out new family rules and agreements to stabilize the family system.