Visual effects (commonly shortened to Visual FX or VFX) is the term used to describe any imagery created, altered, or enhanced for a film or other moving media that cannot be accomplished during live-action shooting.
Essentially, much of the art of visual effects takes place in post-production, after primary image capture is complete. Visual effects can be added to live-action, captured through techniques such as matte painting; rear- and front-screen projection; miniature or forced perspective sets; computer graphic objects, characters, and environments; and compositing of images recorded in any number of ways. The recent explosion in digital tools that make flawless compositing, digital sets, and fully computer-generated characters possible and accessible to moving image makers at all levels has made visual effects a standard part of every moving image maker’s tool kit.
An example of VFX in Deadpool. This video shows the construction of VFX within this crash/fight scene.
Visual effects are digital, made on a computer in post production. Special effects are practical (props, make-up, animatronics, pyrotechnical, etc.), made on set during production. You can also mix them up, like shooting a special effects footage and composite it using visual effects.
Garfield (2004)
BY MATT BARONE
"Since 1978, cartoonist Jim Davis' popular strip Garfield has delighted newspaper readers with its tales of a fat, wisecracking cat underachieving in every way possible. But since 2004, the live-action movie adaptation has been a running joke in Hollywood; hell, even voice actor Bill Murray disses it whenever given the chance.
Why? Because the CGI-rendered feline looks faker than a dog's squeaky toy. Just look at the differences between Garfield and the real-life dog in the above picture. Aside from being ridiculously big for a cat (in that pose, Garfield resembles Godzilla), the film's chubby orange pussy (sorry) is way too digitized."
When I first watched Garfield, I was of quite a young age and therefore, I didn't see a fault with the visual effects used; in fact I was quite amazed. However, now being able to understand that Garfield isn't real and is a form of visual effects, I am able to see how poor the cgi is, in comparison to the real dog as mentioned above by Matt Barone. To improve this they could have either used cgi for the dog too or used an actual cat for Garfield character. This would have elevated any juxtaposition between the two characters.
A good example of CGI is in Dawn of the Planet of the apes - this is explained above.