Ferritin is the storage form of iron. It’s an intracellular soluble protein that is present almost all the cells but mainly present inside Macrophages, Hepatocytes and Intestinal epithelial cells. The ionic iron is toxic for the cell so it’s not left as such, rather it’s put into a special protein “Tin Pack” (container). Such Ferrous containing “Tin packs” are called Ferritin. (Ofcourse, these are not literally the tin packs, It’s just an analogy.) Ferritin consists of a central iron core and surrounding that core is a shell of protein. (Apoferritin). Sometimes in Macrophages especially in the Macrophages of spleen the iron storing capacity of ferritin is overwhelmed, thus aggregates of ferritin forms with their outer protein shell partially digested. Such aggregates of ferritin are called Hemosiderin. Hemosiderin is the insoluble protein and stained with Prussian-blue stain on histological section. Hemosiderin is almost always found in macrophages. while Ferrtin is present in small quantities in all the cells.
I have explained this and many other related concepts in this lecture of Iron deficiency anemia. If you want to watch https://www.youtube.com/watch?v=umUg-heVprU or If you are reluctant to watch such long lecture, skim through the Lecture notes to get the idea.
Another thing, Ferritin is mainly intracellular protein but very minute quantities of Ferritin is released from the macrophages into the blood. This Serum Ferrtin levels directly correlates with the intracellular ferritin (iron stores) level. So Serum Iron levels can be measured to estimate the intracellular storage iron levels.