A 3D model is a 3D digital representation of any object or surface using a 3D modeling software. 3D models can be viewed, scaled, rotated and modified in any angle using compatible 3D softwares.
A polygon is a basic 2D geometric shape made out of straight angles and lines that, when placed together, make up a polygonal mesh, or a 3D model.
The number of polygons in the mesh is called poly-count and the polygon density is the resolution.
The more polygons an object has, the better is its detailing and quality. Thus high-poly objects are best for photorealistic 3D visualizations such as commercial product renders, lifestyle 3D images, VFX and 3D animations. Low polycount (low poly) objects are commonly used in game development or digital e-commerce tools like the 3D product configurator.
Stock 3D models come with a license that gives the buyer permission to apply them according to the terms of use.