[common] To put the newly-added portion of an email or Usenet response before the quoted part, as opposed to the more logical sequence of quoted portion first with original following. The problem with this practice is neatly summed up by the following FAQ entry:

A: No.
Q: Should I include quotations after my reply?

This term is generally used pejoratively with the implication that the offending person is a newbie, a Microsoft addict (Microsoft mail tools produce a similar format by default), or simply a common-and-garden-variety idiot.

One major problem with top-posting is that people who do it all too frequently quote the entire parent message rather than trimming it down to those portions relevent to their reply — this makes threads bulky and unnecessarily difficult to read and arouses the righteous ire of experienced Internet residents (this style is called “TOFU” for “text over, fullquote under”, or sometimes “jeopardy-style quoting”). Another problem is that top-posters often word their replies on the assumption that you just read the previous message, even though their perversity has put it further down the page than you have yet read. Oppose bottom-post.