Abrogation

On another forum a poster uses "Abrogation" to support his contention that Islam seeks to perpetuate war against the entire world. I disagree with this view, but as I don't completely understand abrogation, it's hard for me to effectively refute him. Can someone explain to me what abrogation is, what it applies to and how much importance it has in the Islamic faith?

Thank you.

Thank you for your reply. The way this person uses it is to say that Mohammed's instructions to behave peacefully toward non-believers were abrogated by other commands to kill or convert non-believers and that it remains a duty on Muslims today to kill or convert non-believers. It makes me exceedingly sad and angry that someone would make these claims, but I know too that abrogation is part of Islamic tradition even though I am certain this person is misusing it.

Thanks for your reply. :)