Raycasting is the opposite of raytracing. Raytracing is used in professional 3d modelling software, but it is way too slow for use in games wich require "real time" rendering. Raycasting is simpler, faster, but limited in comparision with ray tracing. If you really want realistic images such as required by architectural renderings, you'll have to use raytracing and wait several hours for the result. But in games you need something that can handle 50+ frames/second, so this can be achieved with raycasting.