These are the basic things about them:
Plasma is the liquid component of the blood.
red blood cells are the red component. It is the part of the blood that carries oxygen from the heart to all parts of the body.
White blood cells are responsible for the body's defences against viruses, bacteria, infection, allergies etc.
Platelets are responsible for coagulation or the clotting mechanism so that if you get injured, you will not bleed to death nonstop. Thanks to platelets, they will clot, harden up and stop the bleeding.