Stem cells are cells that give rise to new cells, and are found in the bone marrow cavities of our bones. Blood stem cells make cells you can find in your blood: red blood cells, white blood cells and platelets (thrombocytes).
Bone marrow is a kind of breeding ground where new, young blood cells are constantly born. As soon as they ripen, they are released into the blood from the bone marrow. This means your blood is continuously renewed.