Advertising is a one-way persuasive communication with a purpose to promote goods or services to potential customers, while sales promotions are marketing activities intended to immediately boost sales. Advertising's primary goal is to build brand image.
Advertisers assume that ads placed on television and radio, in print and on posters and billboards eventually entice consumers to buy their products or services. They schedule advertising campaigns over a particular time period with hopes of exposing potential buyers to their messages multiple times. Advertising is expensive and favored by medium-to-large companies with large marketing budgets.
Sales promotion is divided into trade and consumer promotions. Trade promotions target resellers of a product. Examples of trade promotions include quantity discounts and gifts and other incentives to top sellers. Consumer promotions target the end users of a product or service. They are frequently employed seasonally, around Christmas for instance, and are relatively inexpensive. Their low cost makes them a favorite of small companies; however, large companies use them as well, especially for products in the mature or decline stage of their product life cycle.
A final difference between advertising and sales promotion is that advertising is a long-term strategy, while sales promotion has a short-term focus.