Top posting is replying before the text which you are replying to (and which is usually automatically quoted by your email client).
This example from [Debian wiki](https://wiki.debian.org/FAQsFromDebianUser#What_is_top-posting_.28and_why_shouldn.27t_I_do_it.29.3F) answers to both, this and next question.
It also makes harder to know what previous poster meant if he/she just said "+1" and other people have used different posting style and senders and replies are messed up. This happens often on Lubuntu mailing lists.
At least [Debian mailing lists](http://www.debian.org/MailingLists/#codeofconduct) and [Ubuntu mailing lists](http://www.ubuntu.com/support/community/mailinglists) forbid top-posting in their guidelines. In my opinion other mailing lists should do this too.
<divid="clustrmaps-widget"></div><scripttype="text/javascript">var_clustrmaps={'url':'http://mkaysi.github.com/','user':1040881,'server':'4','id':'clustrmaps-widget','version':1,'date':'2012-09-02','lang':'en','corners':'square'};(function(){vars=document.createElement('script');s.type='text/javascript';s.async=true;s.src='http://www4.clustrmaps.com/counter/map.js';varx=document.getElementsByTagName('script')[0];x.parentNode.insertBefore(s,x);})();</script><noscript><ahref="http://www4.clustrmaps.com/user/bd3fe1f1"><imgsrc="http://www4.clustrmaps.com/stats/maps-no_clusters/mkaysi.github.com--thumb.jpg"alt="Locations of visitors to this page"/></a></noscript>