雖然很多人知道了, 難得看到別人寫好的解釋, 而且還很清楚, 貼一下吧 :p

from:
http://www.w3schools.com/xml/xml_syntax.asp

Do you know what a typewriter is? Well, a typewriter is a mechanical device
used in the previous century to produce printed documents. :-)

After you have typed one line of text on a typewriter, you have to manually
return the printing carriage to the left margin position and manually feed
the paper up one line.

In Windows applications, a new line is normally stored as a pair of cha-
racters:carriage return (CR) and line feed (LF). The character pair bears
some resemblance to the typewriter actions of setting a new line. In Unix
applications, a new line is normally stored as a LF character. Macintosh
applications use only a CR character to store a new line