CGI (Computer Generated Imagery) is also used for 2D and stop-motion animation. But it's 3D CGI animation that has become a popular form of animation. Beginning with Pixar's Toy Story, 3D CGI animation has raised the bar for the images we see on screen.
3D CGI animation is used not only for entire films or TV series, but also for spot special effects. When filmmakers used models or stop-motion in the past, they now can use 3D CGI animation, such as in the first three Star Wars films and Spider-man movies.
Good 3D CGI animation requires specific software programs. These programs used to be available only to studios with lots of money, but with the advance of technology, now someone can create 3D CGI animation at home.
In addition to software programs, you need to employ detailed modeling techniques, shaders and textures to create a realistic look, and build backgrounds and props. Just as much time and work is required in making 3D CGI animation as in 2D cel animation, because the more you build detail into your characters, backgrounds and props, the more believable your animation will be.