VFX is short for "Visual Effects". It's a really broad term used to identify pretty much any imagery that is not created using live action, such as the use of green screens and CGI to insert scenery, objects, or effects into films. A classic example would be the lightsaber effects from the original Star Wars trilogy. VFX and more specifically CGI is used in a lot of modern animation because it is almost always cheaper and faster than creating thousands of individual hand-drawn images. Once an object is drawn and rendered, it can be easily manipulated to do whatever task is needed, within reason. This usually takes quite a bit of practice though. Programs for creating CGI tend to be expensive, but popular choices are Maya, Blender, and LightWave 3D, along with a few others.