Programs that want to install themselves on your computer without your explicit permission. They then reside and collect data about your habits to be sent to them later.
It seems my newest version of Opera may be stopping this now. It doesn't change the nastiness of this site to unsuspecting people not properly protected. It actually should be a jailable offense for the owner of the site to support this type of nefarious third party software (third party because I'm sure they will deny complicity).

---Al