This profession/hobby/calling is very unique, mainly in it's combination of artistry and technology. Someone who is just a great artist cant do it without the technological background while at the same time it doesn't matter how much technical knowledge you have if you have no artistic ability or talent. A 3D artist is unique in the combination of the necessary skill and flare needed to make a stunning 3D Rendering or Animation.
The tasks of 3D Artists can be broken down into a few:
Modeling is the creation of 3D Mesh to form shapes. These may be the walls, windows, doors, roofs and solid features of a building as well as smaller things like furniture and decorator items. In fields outside of architecture it could be a character, a car - ANYTHING.
Texturing is the application of surfaces to the 3D mesh to simulate real materials. Textures may be made from photographs or can be totally abstract. A finished surface may actually be a combination of many textures that simulate features such as roughness, transparency, reflection, diffusion, specular, illumination, etc.
Lighting is the placing of 3D light sources within a scene to simulate real life. Just like reality there are many different kinds of lights that also have many different features. Complete books have been written on 3D lighting, as have on other parts of a 3D artist's job.
Compositing is the blending of real life video or photography with the 3D scene/objects. A movie or finished rendering doesn't need to be all 3D, in fact sometimes it is best if it is only partial. Camera tracking is a perfect example of this.