Kissing where one partner effectively ends up deepthroating the other one's tongue (with or without the lips touching), or less often some kind of out-of-mouth kissing (like in the first example, post #513445). By virtue of being "oral-to-oral interaction with tongue", this is broadly counted as french kissing. The use of long tongues is basically a given, and excessive amounts of saliva/drooling are common. A good rule of thumb is: If you can do it with 2 normal humans, it probably isn't an extreme french kiss.
See also