Both hard surface and organic modelling are at their core still polygonal modelling. Although different techniques are used between the two.
In a nutshell, the difference between organic and hard surface modelling is exactly what the names imply. Organic modelling is usually creating 3D models of living creatures such as people or animals. Hard surface modelling is usually focused on inorganic objects such as buildings or furniture.
Though many of the principles of modelling between the two are similar, it takes a lot of practice to be able to master both or to specialise in just one.
Another important thing to note is the line between these two categories is often disputed.