Tutorial by Examples

Raycasting means throwing a ray from the mouse position on the screen to the scene, this is how threejs determines what object you want to click on if you have implemented it. Threejs gets that information using an octree, but still in production you may not want to compute the result at each frame ...

Object picking using Raycasting might be a heavy task for your CPU depending on your setup (for example if you don't have an octree like setup) and number of objects in the scene. If you don't need the world coordinates under the mouse cursor but only to identify the object under it you can use GPU...