When most people use word processors, they think in terms of just making a document look good on screen and making it print out the right way. When you use styles, you think not only in terms of making a presentable document, but also in terms of making a document that is consistent, maintainable, and easy to edit in the future. You do this by implementing a very powerful document design strategy: separating form from function by using styles.
With styles, you don't have to specify the form of the text as you type (such as 14 pt., Arial font, bold, every time you type a heading). Rather, styles let you just specify what the function of the text is ("this is a heading"), and the word processor takes care of the text's presentation.
To use styles, you begin by looking at the text that makes up your document. Here you define the different classes of text that your document contains, separating each of them by their function. For instance, a typical document might include styles for headings, body text, quotes, page numbers, headers, footers, etc.
Next you tell the word processor how each of these classes of text should appear on the page. (All headings should be 14 pt., Arial, bold; all body text should be 10 pt. Georgia, etc.)
The advantages of styles are