A 3D model is a mathematical representation of something three-dimensional.
3D models are used to portray real-world and conceptual visuals for art, entertainment, simulation and drafting and are integral to many different industries, including virtual reality, video games, 3D printing, marketing, TV and motion pictures, scientific and medical imaging and computer-aided design and manufacturing CAD/CAM.
Some 3D models are constructed from NURBS (non-uniform rational b-spline), smooth shapes defined by bezel curves, which are relatively computationally complex. The typical base of a the model is a 3D mesh; the structural build consists of polygons.
When models are created for animation, they require careful construction because the polygon layout can create issues in unusual deformations. The models also require the construction of a skeleton and the painting of weights, which define the texture and polygon deformation of the model under movement.
Some 3D models define surfaces through shaders, programs that mathematically define color, lightplay and other surface characteristics. Other models define color, specularity, surface texture, and light emission through a series of 2D image files called maps, especially those used in games where raster graphics are needed to deliver real-time frame rates.
A more recent development in 3D modeling is reality capture, which uses remote sensing technology such as Lidar to capture complex forms quickly and accurately. Reality capture may be used in combination with 3D printing for an end-to-end process known as reality computing.
See a beginner's tutorial on creating a 3D model: