Top-posting means that you write your reply _above_ your quoted text.
Just like Mike Berger did in this thread without valid arguments. It's
his madness, not mine nor yours. So please write your reply _below_ the
quoted text, like I did. It's the correct way, and called
bottom-posting.

So just think:

A: Top-posting.
Q: What is the most annoying thing in the newsgroups?

Correct way:

Q: What is the correct way?
A: Bottom-posting.

In other words, think you probably didn't write your answers above the
questions in exams in school. Not more complicated :-) I see some
newbies don't know that, and moreover some programs have the cursor in
the beginning of the text field (where the quote will be automatically)
as default, and then those newbies just write there :-(