What are 3D Shaders?

Link: http://en.wikipedia.org/wiki/Shader

Shaders: A shader is a computer program.It is use to calculate rendering effects on graphics hardware with a high degree of flexibility.

Basically, there are different types of shaders Pixel, Geometry and Vertex Shaders.

Pixel Shaders : It is also known as fragment shaders . I t is use to compute the colour and the other attributes of each of the pixel.

Geometry Shaders: It can be said to be a new type of shader.It can generate new graphics primitives. They are like points, Lines, shape in general.

Vertex Shaders: It is a shader that will generate when vertex is given to the graphics processor. "The purpose is to each vertex in 3D position in virtual space to the 2D coordinate at which it appears on the screen (as well as a depth value for the Z-buffer)." Quote from paragraph.
It can also generate/ manupilate properties like colors,position,textures...but not to create a new vertex/vertices