How exactly does IUD work?

Hi,

I'm currently on a combination pill and really hate it because I can't stand the thought of synthetic hormones running through my body. I'm in a stable, monogomous relationship, never had a child, and I really don't want kids within the next three years. I've been researching alternatives and am interested in the copper IUD since it has no hormones but I'm not exactly sure how this prevents pregnancy.

I checked the tags and didn't find a specific answer to this so I'm hoping someone can help.

I read that the copper creates an uninhabitable atmosphere for sperm and, in the event this occurs, prevents a fertilized egg from implanting in the wall of the uterus, correct? My mother thinks the IUD is basically abortion birth control. I know I need to make this decision on my own but I'm not quite sure when conception actually occurs--at fertilization of the egg or at implantation in the uterine wall?

I'm just trying to do as much research as possible before I speak to my doctor and want to be clear on everything.

Thanks! :-)