What is Illustration?
    If someone asked you, "Have you been to any good restaurants lately?" you probably wouldn't answer "Yes" and then immediately change the subject.  Most likely, you would go on to illustrate with examples.  Perhaps you'd give the names of restaurants you've enjoyed and talk briefly about the specific things you liked: the attractive prices, the tasty main courses, the pleasant service, the tempting desserts.  Such examples and details are needed to convince others that your opinion—in this or any other matter—is valid.  Similarly, when you talk about larger and more important issues, people won't pay much attention to your opinion if all you do is string together vague generalizations: "We have to do something about acid rain.  It's had disastrous consequences for the environment.  Its negative effects increase every year.  Action must be taken to control the problem."  To be taken seriously and convince others that your point is well-founded, you must provide specific supporting examples: "The forests in the Adirondacks are dying"; "yesterday's rainfall was fifty times more acidic than normal"; "Pine Lake, in the northern part of the stea, was once a great fishing spot but now has no fish population."
    Examples are equally important when you write an essay.  It's not vague generalities and highfaulting abstractions that make writing impressive.  Just the opposite is true.  Facts, details, anecdotes, statistics, expert opinion, and personal observations are at the heart of effective writing, giving your work substance and solidity.

Nadell, Judith, John Langan and Linda McMeniman.  The Macmillan Reader. 4th ed.  Boston: Allyn and Bacon, 1996, 239.