SKIN Shader is an example of vertex shader,geometry shader or fragment shader?

I wanted to know shaders like skin shaders are type of vertex shaders or geometry shaders etc or they are different than this?

They are a combination. When manually writing shaders you tend to do a couple of things in one (for example the vertex shader) then pass that to the next (often the fragment shader) until you get the end result you like. When you create a material in Unreal the shader compiler automatically determines how they are processed so you generally do not have to worry too much about it.

In addition skin is a material type in Unreal, so depending on your node network you’ll generate different actual shaders, that might use more or less hardware shader features. For example turning on tesselation will require the use of the hull shader in hardware.