Your arteries carry blood from your heart out to the rest of your body. They have very thick walls which allow them to withstand the immense pressure created as your heart pumps blood forcefully. As blood travels outward, the arteries become smaller and smaller until eventually the blood enters into what are known as capillaries. Capillaries are so tiny that blood usually can only get through one cell at a time. It is within the capillaries that oxygen is taken in by the cells, and waste, such as carbon dioxide, is released into the blood.
Eventually the capillaries get bigger and bigger, and then the blood enters into veins. As the blood travels back to the heart your veins get larger and larger.