3DCG in general is used for many things: special fx, quick iterations, sculpting, games, simulation, animation, rendering.
Suites like 3ds Max, Maya, Lightwave, Softimage, and Blender can do most anything. You can bang out some simple models, render them out to a image file and paint over it. This is a common technique among conceptartists. There are also images that are completely made in these programs and 2D imaging software is used for textures and composition. These are used to create the 3D models and animations for games too. Big movie studios use them for special effects or animation. Pixar has their own software for rendering but I think they might use Maya in conjunction with it.
There are specialized pieces of software. Sketchup is for architectural visualization or engineering. Zbrush is for sculpting and creating highly detailed meshes for use in other programs.
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