Advertising and Promotion are tactics used to reach and influence the market. But they typically have different objectives and roles in the marketing mix. Understanding these different objectives helps us to employ the tactics in a more effective manner.
Advertising is generally targeted at end users (as opposed to the channels of distribution). It can be used for many purposes, including establishing awareness, providing information for knowledge, and creating brand loyalty. In its knowledge role, advertising can communicate a positioning (or modify a positioning) and even promote new uses of a product (think "Orange Juice isnt just for breakfast anymore").
By focusing on-end users, advertising can indirectly influence the channels of distribution. This is because if advertising is effective in generating interest in a product, the channels may be more interested in carrying the product. When this happens, advertising "pulls" the product through the channels. For this reason, we tend to call advertising a "pull strategy".
Sometimes advertising is used to enhance a companys "goodwill", so we feel better about a company. While not directly related to a specific product, these types of ads are useful in building an overarching brand identify.
Promotion is a general term. It is so general, in fact, that most communications that are not strictly advertising (paid and non-personal) are characterized as promotions. But this distinction is blurry at times, so looking for a clear definition is useless. Its better to just recognize that there are many types of promotions.
Some promotions are aimed directly at consumers (e.g., special events, sampling, rebates, coupons, etc.). The use of these promotions can typically be tied to the hierarchy of communication effects model.