EARS FORWARD: Signals the approach of food, sex or a predator. (Humans fall under the last heading.)
ONE FORWARD, ONE BACK: Sometimes it is a case of watching you and the road. If they turn both ears around then you have gone from a curiosity to an annoyance. Relocation is imminent.
EARS SIDEWAYS: Actually it is a genetic anomaly or the ears are broken. Otherwise, it doesn't mean a damn thing.
EARS FLAT BACK: Unabashedly pissed off. Good time to walk away quietly.