Wow!
River Raid was simple. You fly a plane that shoots down obstacles as the screen scrolls vertically. You have to keep your fuel supply up by flying over fuel stations. Make sure you don't crash into the land areas or other objects. Other than that, the game sped up as you reached the end.
All in all this was not a long game. The nice thing about it is that it inspired future games similar to it. The most interesting ones that come to mind are side-scrollers like Gradius III and other vertical scrollers like Ikargua. I played this for hours and even though it was short, it was difficult to make it to the end.