Veins carry blood back to the heart. They carry deoxygenated blood away from the body. The blood returning from the body is at a much lower pressure than that being pumped from the heart. Therefore veins do not have to be as strong as arteries. Veins are wider than arteries and have much thinner walls. Look at the image below to see the main difference between arteries and veins.
Cross section of Vein
As the blood pressure is low in veins there are mechanisms which help to keep the blood flowing in the right direction and stop it from pooling at your feet. Veins have valves which prevent any backflow of blood. These valves only open in one direction. As blood flows along the vein it pushes the valves open, the valves then close to stop any blood from going backwards. Veins are often surrounded by lots of muscle and when these muscles contract they squeeze the veins, further help to push the blood in the right direction, back to the heart.