A causal relationship is a relationship in which one thing causes another thing to happen or change.
(State Educational Technology Directors Association. 2008. "Scientifically Based Research Glossary." http://www.setda.org/web/guest/glossary)
For example, if a particular education program causes test scores to rise, there is a causal relationship between the program and higher test scores.
In this example, if test scores go up following the implementation of the education program in question, this alone does not demonstrate the existence of a causal relationship. There is only a causal relationship in this instance if test scores increased because of the existence of the program.