Product advertising is a very broad category which can include adverts placed in a variety of different media, all with the goal of promoting a specific product or service, usually to a targeted audience.
Consider the product and its intended market; what forms of media will be most likely to reach the target market and help raise awareness of the product in question. The answer will depend on the nature of the product, at least in terms of narrowly focused target marketing, but it often pays to place product advertising in media outlets which have a general audience as well (for example TV and radio adverts, as well as mass market print media such as newspapers).
The idea is to convey the merit of the product and to give the consumer an idea of the benefits to be gained by using this particular product. Product advertising should get across the message that the product will make the consumer's life easier, more enjoyable or somehow imbue them with higher status (as in the case of product advertising for luxury products and other big-ticket items). Since product advertising can be such a complex matter, companies will leave this work to the professionals; a skilled advertising agency which has an in-depth knowledge of various forms of media, consumer psychology and a knack for getting your message across to consumers in a way which will ensure the greatest impact and market penetration.
An advertising agency can also be invaluable in terms of having media contacts which allow them to negotiate a more favorable rate for advertising placements. This allows you to reach more potential customers with your product advertising in a cost effective manner; and giving you more time to focus on the all-important task of serving your customers.