Blood is a complex fluid that circulates through the body, transporting nutrients, waste, hormones, gases, and defending the body. It consists of plasma and blood cells suspended in plasma. Blood types are categorized by the presence or absence of antigens A and B on red blood cells, and the presence of the Rh factor, resulting in four main blood groups: A, B, AB, and O. Donating blood helps patients but donors must meet eligibility criteria like age, weight, frequency of donation, and health to safely give blood and prevent complications.