Blood contains formed elements. Erythrocytes, leucocytes, and platelets are collectively known as formed elements. 

i. Erythrocytes - They are also called Red Blood Corpuscles (RBCs). They are the most abundant elements present in the blood. RBCs are formed in the red bone marrow in the adults. They are devoid of a nucleus.
Their main function is the transport of the respiratory gases.
ii. Leucocytes - They are also called White Blood Corpuscles (WBCs). They are nucleated and less abundant as compared to the RBCs. The WBCs are further divided into granulocytes and agranulocytes. The granulocytes are of three types neutrophils, eosinophils and basophils. Monocytes and lymphocytes constitute the agranulocytes. The lymphocytes are further 
The main function is to provide immunity to the body against the pathogenic organisms. 

iii. Platelets -  They are also known as thrombocytes. Platelets are small irregular bodies present in the blood. They contain essential chemicals that help in clotting.
The main function of platelets is to promote clotting.