[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.