While both are visual media, to sum it up in a nutshell, 3D and CGI usually refers to movies and or video games. Basically moving images. Graphic Design deals primarily with print media or static images. This is NOT the be-all-end-all definition defining the two - I just think it will help you differentiate between the two
3D and CGI doesn't really translate well to the printed page. If, in fact, at all. Graphic design deals with placement of text, colors, design on a page. How to guide the eye to make the "reader" get the sign from the designer. How to convey a message to the end user. etc . . .
Graphic Designers are the ones that help layout magazines. The ones that design corporate logos. The ones that print all sorts of labels on food items, wine, beer, etc. . . Graphic Design is usually also called 2d design. It's flat - if that makes sense.
3D and CGI frequently uses moving images to convey their message. Almost all of the movies that you've seen within the last few years deal with 3d and or CGI. CGI applies to say . . . Transformers, and how the bots transform. 3D deals with how the image "jumps" out at you. And how your eye reacts to images behind those glasses you get at the movies.
3D images deal with wireframes, where as 2d images won't. Think of it as a side scroll game (super mario bros) vs.a first person shooter (left for dead). IF a graphic designer were to work on a game like "Left for Dead" he/she might design the background of areas the user might encounter. Might also design "how" the characters look or the background looks. But more than likely will design the box, how the manual is put together visually, and any advertising materials needed to push it out.
CGI is a really, really broad category as it touches all forms. CGI stands for "computer generated imagery". CGI can be both 2d and 3d. After all, you can create a document entirely in photoshop in 2d and it can be considered CGI. OR it can be created in Maya 3D and still be called CGI. The best idea is to kinda "forget" CGI and concen yourself with is it "3d or 2d"?
This is a long-winded approach to help you understand. It's not the "final" answer and I'm sure there's others that could explain parts of this better. However, I'm hoping the 2d and 3d aspects will help you along the way.
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