Hi, What is the difference between 3D hard surface model and organic mesh models. I guess it is only the smoothness. Can somebody give me more idea about it please. Regards

There are at least two differences between the two.

One is the lighting/shading. Organic models usually have a greater diffuse light and subsurface scattering contribution, and "hard" surfaces such as metal usually have more specular contribution and little or no subsurface scattering. Exceptions from that rule are "hard" surfaces like jade or marmor, which have a lot of subsurface scattering or rough, dry rock, which is almost only diffusely lit.

The other difference is whether vertices are static and the object is a rigid body, or whether the vertices can move along. There is further a distinction of volume preserving and non-volume preserving objects. Most "organic" things are volume preserving (jelly), but allow vertices to be displaced, most "hard" things are rigid (rocks). Some things are not volume-preserving (sponges, balloons).
Needless to say, computing deformations is not free, so many games opt to not implement them. Due to dedicated physics hardware and GPGPU, it's getting better, though.

thanks ever so much samoth for your help. I appreciate it.

Regards