The elevation of an object is it's height above sea level. Sometimes elevation and altitude are using interchangeable, however, altitude is the vertical distance between an object and the earth's surface.
So if for example, a plane was flying over a mountain range, the plane's altitude would be the verticle distance from the plane to the surface of the mountain range whereas the elevation of the plane would be the height of the plane above sea level.
Relief is the difference in height elevation between geographic features. We could for example use relief to describe the elevation, direction, and angle of slope of a mountain range. We could also simply say that relief is the opposite of flatness.