Vein

The structure of a vein is as follows:

The walls (outer structure) of veins consist of three layers of tissues that are thinner and less elastic than the corresponding layers of aerteries.
Veins include valves that aid the return of blood to the heart by preventing blood from flowing in the reverse direction.

The functions of veins include:

Transport blood towards the heart;

Carry De-oxygenated Blood
(except in the case of the Pulmonary Vein);

Have relatively wide lumens (see diagram above);

Have relatively less muscle/elastic tissue;

Transports blood under lower pressure (than arteries);

Have valves throughout the main veins of the body. These are to prevent blood flowing in the wrong direction, as this could (in theory) return waste materials to the tissues.