Fields are an nice, efficient way to include frequently updated information about your documents automatically. There are a ton of things you can include such as your document file name, the last date your document was saved, the total number of pages, or the initials of the user who created the document, etc. Check out this nice summary for a more complete list of what you can include.
Regardless of what type of info you choose to include, Word will automatically update your fields as changes occur. Many times you may want to put these fields into a header or footer, but you can also put them anywhere else in your document too.
How It Works
1. From the Insert menu, select the Field… option.
2. In the resulting dialog box, simply select the field you want to use, apply any options for the field properties (if applicable).
3. When you click the OK button, Word will insert the field wherever your cursor is located.
See It In Action
In Word 2010 field are on the Insert tab under Quick Parts » Field Names. See the details here.