This is a short excerpt from Wikipedia.

Media Buying is a sub function of Advertising management.

Media Buyers

Media Buyers are individuals responsible for purchasing time and ad space for the purpose of advertising here.^[1] When planning what to buy, they must evaluate factors based on but not limited to station formats, pricing rates, demographics, geographic, and psychographics relating to the advertisers particular product or service objectives. The Media Buyer needs to optimize what is bought and that is dependent on budget, type of medium (radio, internet, TV, print), quality of the medium (target audience, time of day for broadcast, etc.), and how much time and space is wanted.

Media Buyers can purchase spot, regionally, or nationally. National Media Buyers might have to factor in determinates based on a state by state basis. Rates, demand of leads, space, and time, and state licenses will vary from state to state. National Media Buyers will need National Media Planning to generate National Media Marketing strategies and National Media Advertising that can be adaptable from state to state but also work on a national level.

There is an apparent distinction between General Marketing Media Buyers and Direct Response Media Buyers. General Market Media Buyers enact or actualize media plans drawn up by media planners. They negotiate rates and create media schedules based on a media plan constructed by a Media Planner. Through the Media Planner, General Market Media Buyers rely on published cost per point guides which in actuality, are often based on hypothetical benchmarks, and rather outdated models. An experienced Direct Response Media Buyer knows what stations generate a specific quantity of response and knows within reason, the break even point of the expenditure versus the return. With that information, the Direct Response Media Buyer is efficient in negotiating a functional rate and in purchasing media from the appropriate stations. The Direct Response Buyer attaches unique phone numbers to each station they purchase media from and track the sales, and make adjustments to the media plan and schedule as necessary to optimize results. With these differing methodologies, Direct Response Marketing can be considered a specialized arena. Few advertising and marketing agencies are qualified to support clients in their Direct Response efforts.

More resources:

http://en.wikipedia.org/wiki/Media_buying

http://en.wikipedia.org/wiki/Advertising_management

http://en.wikipedia.org/wiki/Media_planning