A pen is something that you use to write or draw with. Pens are filled with ink. Generally once you have drawn or written something in pen, you can't rub it out. You can rub out drawings in pencil.
Pollen is made up of fine grains of male genetic material. Bundles of pollen are held inside flowers on the end of stamens, which you can clearly see in the picture. As insects or birds come to the flowers to eat nectar, they brush against the pollen and carry it to the stigma of another flower.
A stamen is the part of a flower that makes pollen. They are usually in the center of the flower, and have stalks called filaments with the pollen produced on anthers at the top of the stalks.