Media buying is about the purchasing of media advertising space or slots (in both traditional as well as new media).
For people starting off in this area of media, media buying will often be combined with media planning (understanding who the audience is, understanding the different types of media options available and their suitability or relevancy to a particular brand, and so on).
The rise of digital media has made the task for media buying that much greater. When media buyers only (relative to what we now have in new media) had to deal with TV, radio and print, it was much easier to get to grips with the technology and the people who worked in these areas of traditional media. But with new media things are very different. The spaces and slots for advertising are far more widely dispersed and it is much more difficult to get to know the people behind them, and to build up working relationships, and so on.
Media buyers are employed in the following sorts of organizations: in general media agencies (that carry out media planning, media buying, sales, marketing and so on); in agencies focused on digital media; as well as in both traditional and integrated advertising agencies.