Cluster analysis is a set of techniques designed to answer the question “How do my subject cluster together on the variables that I have?” This is a question that is often interesting, but is too rarely asked, because many applied researchers have not heard of the methods involved.
Suppose, for example, you had data on the votes of members of a congressional or parliamentary body. Cluster analysis would allow you to ask whether there were groups of politicians who voted similarly. In systems with strong party voting, you could do this within particular parties. Or, you might look at the whole body, and see if some members of particular parties were outliers within their party.
Or suppose you had the answers of a group of subjects to all the questions on a questionnaire or personality measure or test of ability. You could use cluster analysis to see if people grouped together on their responses to some items. Perhaps some particular set of items identifies a key group of people.
There are a great many methods of cluster analysis, but they all attempt to answer this basic question.