Red blood cells are red because of the haemoglobin/hemoglobin in them. This stuff is responsible for carrying oxygen from your lungs to various parts of the body and for carrying part of the carbon dioxide back (most of the carbon dioxide is dissolved in plasma).
Normal red blood cells are 7 microns in diameter and have a biconcave appearance. Red blood cells are made in the bone marrow of long bones. Mature human red blood cells have no nucleus.