Plasma
More than half of your blood is comprised of plasma: a clear, yellow liquid which is mostly water. The plasma contains proteins and other substances that make the blood coagulate when you hurt yourself.
During your blood's journey through your body, its circulation, the plasma carries with it nourishment that your body needs and removes any waste that your body wants to get rid of.
Red blood cells
There are milions of red blood cells floating around in the plasma. These carry with them oxygen from your lungs to all parts of your body and bring back carbon dioxide to the lungs, where it is exhaled. (Even your plasma carries some carbon dioxide.) Red blood cells are created in your bone marrow. They only last for a few months, so your marrow is always busy making new ones.
White blood cells
The white blood cells are your body's bodyguards. Whenever dangerous bacteria enter your body, the white blood cells rush there, surround the bacteria and kill them. New white blood cells, which are mostly created in the bone marrow, replace those that die in battle.
Platelets
The platelets are there to prevent too much blood from running out, for example when you cut yourself. They gather at the wound, bind with certain substances in the plasma and form a layer that seals the wound. This layer dries to become a crust, a scab. Under the scab, your skin heals and after a while the scab falls off. (Unless you've already peeled it off, that is…)