Clustering is a type of prewriting that allows you to explore many ideas as soon as they occur to you. Like brainstorming or free associating, clustering allows you to begin without clear ideas.
To begin to cluster, choose a word that is central to your assignment. For example, if you were writing a paper about the value of a college education, you might choose the word "expectations" and write that word in the middle of your sheet of paper. Circle "expectations," then write words all around it--words that occur to you as you think of "expectations." Write down all words that you associate with "expectations," words that at first may seem to be random. Write quickly, circling each word, grouping words around your the central word. Connect your new words to previous ones with lines; when you feel you have exhausted a particular avenue of associations, go back to your central word and begin again.
For example, "expectations" might lead you to consider "the social aspects of college," which may lead you to consider "career networking." You may then find yourself writing down words that compare the types of jobs you might get through career networking. You may end up asking yourself questions such as "What sorts of jobs do I want? Not want?" Have fun with this exercise; even silly questions can open avenues to explore, such as "What if I ended up waiting tables at Buddy's?" "Would I rather be a lion-tamer or an accountant?" "What about my brilliant career as a stand-up comedian?"
Some words will take you nowhere; with other words you may discover that you have many related words to write. Random associations eventually become patterns of logic as you look over your work. After looking over the clustering exercise above, you might conclude that you want an exciting career as a performer of some type, rather than a job in the service sector or behind a desk.
Now your sample paper about the value of a college education has some focus: how you expect college to lead to an interesting career that involves creativity, skill, and performance. You might then want to return to the phrase "Job Skills" and develop that part of your cluster, noting the skills that you'd need to reach your ideal career.
Clustering does not take the place of a linear, traditional outline; but, as the example shows, it allows you to explore ideas before committing them to a particular order.