Widows. orphans, rivers & lakes.

A widow, in printing terms, is when the last word of a paragraph is the first word on a page. It doesn't look nice.

An orphan is similar, but in the middle of a page. Ending a paragraph with a single word on a line is not attractive.

Lakes & rivers are easiest to spot if you turn the text upside down.

A "lake" is formed when adjascent word spaces in consecutive lines amalgamate to form a gap in the text.

A river is formed when such gaps snake across a page.

Getting rid of these really improves the appearance of text.