Definition
In 3D computer graphics, 3D modeling is referred to the process of developing a mathematical coordinate-based representation of any surface of an object. The objects can be inanimate or living and they should be represented in a simulated 3D space via specialized software and by manipulating their edges, vertices and polygons. A 3D model is basically a model of any object, character, environment, or props that are created in a 3D environment. 3D software is used to create these models.
Most animators start the 3D animation process by creating a 3D model first. This process of modeling requires thorough observation, correction, and alteration of 3D figures with the help of the software. You not only need to be a good animation artist to make a 3D model, but you also need to be excellent at using the various technical 3D software and additionally be great at Anatomy as well. 
Uses of 3D Modeling in Animation
For 3D modeling to be used in animation, the models need to look pleasing and as realistic as possible. They should also be functioning according to the requirements of the animation project and be made using the resources at hand. 3D models are used for a variety of purposes in animation.
You will find them being used in video games, commercials, movies, and songs. In addition to that 3D models are also used in the fields of Architecture, Marketing, Designing, Medicine, and Engineering to understand the various phenomenon of these fields. Experts in these fields also rely on 3D models to plan, design, and execute their work better. 
Additional information
3D modeling is now an imperative part of the contemporary digital media landscape. Almost every advertisement you see on TV now uses 3D modeling in the production process. These models are incorporated so subtly that the common man might find it hard to spot them even.
Movie Modelers and Animators use 3D models for special effects whereas Game Designers often use them to create characters and objects. In the medical field, 3d models are used for treatment purposes and scientific advancements. Furthermore, the Engineering and Designing discipline uses 3D modeling for creating sample/dummy projects to imitate their final results.
