[ivank]

The filter list will block the div by id, class, or by its position among some other containing element.

[knd775]

But how does it know that it is an ad? Some advanced heuristics?

[ivank]

It's all manual. Someone sees it and submits the item into the EasyList blocklist: https://easylist.adblockplus.org/ - it looks like http://forums.lanik.us/ is used for reporting items to block or unblock.

[jordigh]

Filter lists are curated by humans. It's an arms race between two smart groups.

[zeeshanm]

Probably adblockers keep a dictionary of filters for blocking native ads.