Shaders ?

Blitz3D Forums/Blitz3D Programming/Shaders ?

Can anyone provide a simple definition of "shaders" ?

I don't mean, a manual description... I mean just
say it with your words.

And may be, at the same time you could explain me this:
For me, shaders are just a way of blending a texture
in a polygon according to a certain mathematical equation.
So for me, Blitz3d does have shaders, and multiply x2, or
dot 3 blending, or additive blend, ARE shaders ... so
my question could be, if we would have a way of
entering custom "blending equations" inside b3d, could
this be called as shaders?

I'm probably wrong about most of I have said, so please
explain it easily :)