3D modeling is the process of creating a 3D representation of any surface or object by manipulating polygons, edges, and vertices in simulated 3D space. You can see the results of 3D modeling in movies, animations, and video games filled with fantastical and imaginative creatures and structures.
Organic Modelling is the process of creating smooth, flowing curvature and natural organic meshes such as to describe nature and living creatures and typically non man-made objects. Primarily this involves subdivision surface poly modelling and / or sculpting.
Sculpting, which is primarily used for character design, involves modelling with millions of polygons in a very artistic, non-technical way via direct manipulation of a surface with a range of sculpting 'brushes'.