Ideographic explanations are explanations of specific events, phenomenon or behaviours which are sought in the careful examination of specific preceding events. For example, Why did Mary murder the butler? Or, What caused World War I? For the most part clinical psychologists and historians are interested in ideographic explanations.
Other disciplines, like sociology, are interested in explanations of classes of events or behaviours and seek these in a careful examination of a few general categories or classes of preceding events. For example, why do men murder their partners? Or, What are the causes of international violence? These explanations are known as nomothetic explanations.
Idiographic vs. Nomothetic
However the whole issue of whether a trait exists in all people to a greater or lesser degree is complicated by different views of the trait perspective. Idiographic: people have unique personality structures; thus some traits (cardinal traits) are more important in understanding the structure of some people than others.
Nomothetic: people's unique personalities can be understood as them having relatively greater or lesser amounts of traits that are consistently across people.
The Idiographic view emphasizes that each person has a unique psychological structure and that some traits are possessed by only one person; and that there are times when it is impossible to compare one person with others. This viewpoint also emphasizes that traits may differ in importance from person to person (cardinal, central and secondary traits). It tends to use case studies, bibliographical information, diaries etc for information gathering.