Well since it happened while you were driving the car and since the motor still runs it has to be narrowed down to just a few items. Most likely something with the valve train as mentioned. Probably a bad timing chain or maybe some bent pushrods. Could also be a bad cam or bad lifters. Might be something inside the distributor that came apart or some cracked sparkplugs or somthing else odd but I'd start with pulling the valve covers.

You didn't mention any engine noise though and typically bent pushrods or punched rocker arms will make a bunch of noise.