Promo units (or promotional units) are used on a site to promote certain areas, products or services offered. They are often small graphics that can be controlled by the site owner and changed in the content management system where appropriate.
On eCommerce sites, there is usually space within the left and right navigation to use promo units to help reinforce the sale.
Also promo units can be used on the home page with an automatically changing image to help promote key products.