Here is a paragraph of class="nowrap" with property white-space: nowrap. Note how white space in each line is collapsed, but no line breaks are allowed. The only line breaks that appear will be those produced by <br />'s.
Here is a paragraph of class="pref" with properties: white-space: pre; and font-family: monspace; Note how both white space in the lines and line breaks are preserved.
Here is a paragraph of class="pref" with properties: white-space: pre; and font-family: arial,sans-serif; Note how both white space in the lines and line breaks are preserved. However, things don't line up vertically, when there is text to the left of a vertical alignment line, because the font is not monospace.