If you’re a cage (car) driver you might not know about the biker’s nod or wave. There seems to be a fairly universal fraternity amongst bikers, and bikers tend to acknowledge each other when they pass. Some bikers are more snobby about it than others, and will only acknowledge the right kind of bike – for example a Harley rider might only acknowledge other Harley riders, and almost no-one acknowledges scooter riders. But in genera,l most bikers are a friendly bunch and will acknowledge most other bikers, unless they are in the middle of something like changing gear or negotiating a tight bend.
In the UK, the common acknowledgment is a nod, in France it’s an extended boot and in the US it’s a wave or salute.
In the UK, we drive on the left, unlike most of the rest of the world, which makes hand signals to on-coming riders difficult, since the right-hand is on the throttle, so we nod at each other. When you’re driving on the right, the left-hand is free most of the time (it’s used for the clutch) and can be used for hand-signals.
The commonest signal here in the US seems to be left arm pointing downwards with 2 fingers extended in the peace sign. I found a couple of amusing articles on Motorcycle Etiquette: How Not To Wave Like A Dork! and Secret Motorcycle Hand Greetings: Revealed!