From the Word2007 help file (not sure what earlier versions have to say):
A professional-looking document never ends a page with just one line of a
new paragraph or begins a page with only the last line of a paragraph from
the previous page. The last line of a paragraph by itself at the top of a
page is known as a widow. The first line of a paragraph by itself at the
bottom of a page is known as an orphan.
1.. Select the paragraphs in which you want to prevent widows and orphans.
2.. On the Page Layout tab, click the Paragraph Dialog Box Launcher, and
then click the Line and Page Breaks tab.
3.. Select the Widow/Orphan control check box.
Note This option is turned on by default.
Greg Maxey/Word MVP
For some helpful tips using Word.