First of all a hello!

Hope it does not bother anyone reading my english, my german knowledge quite bad so ...

The question I have are related to vertex shaders. What are vertex shaders and what are they good for?
I know a vertex is one point of a triangle and I also know that graphic-cards scale down every higher-level-primitives into triangles, but why do I need special shaders for this.

Ohh another thing which is quite not so clear for me: Fragment-Shaders = Pixel-Shaders?

Thanks in advance, lg FiReDoG