Generally, promotion is communicating with the public in an attempt to influence them toward buying your products and/or services.
How does promotion differ from advertising? Promotion is the broader, all inclusive term. Advertising is just one specific action you could take to promote your product or service. Promotion, as a general term, includes all the ways available to make a product and/or service known to and purchased by customers and clients.
The word promotion is also used specifically to refer to a particular activity that is intended to promote the business, product or service. A store might advertise that it's having a big promotion on certain items, for instance, or a business person may refer to an ad as a promotion.