Answer to Question 1
A relationship study involves establishing which variables are reliably related to one another. A prediction study involves using a predictor variable to predict a criterion variable.
A relationship study might precede a prediction study because we need to understand which variables are related before we can make useful predictions based on those relationships.
Answer to Question 2
Most psychological research with people involves undergraduate psychology students, the majority of whom are female. So we are likely to be able to generalize to other young, educated women, although it isn't always clear if we can generalize to men or to people younger or older than the female college students. For some measurements, the participants we use may be representative of many other people, but for other measurements (e.g., attitudes), the participants may not be like older or younger people or men, or even female students at other schools or in different parts of the country or in other countries.
The issue is complicated because, for some measurements, our participants produce very similar results to many other groups (e.g., speed of learning one type of material versus another), whereas for other measurements (e.g., attitudes toward abortion), participants in a single location may not be like others.
Model organisms used to study the genetics of development
Relationship of the electrocardiogram to electrical conduction in the heart
Leonardo da Vinci, Study of Human Proportion: The Vitruvian Man.
Relationship between Economic Openness and GDP per Capita
Heartfelt Forgiveness Study
The Olestra study followed a sequence of steps