What is above the line advertising? The best examples are most television, internet, and radio commercials; the bigger the event, the more above the line (think Superbowl). AoL advertising foregoes targeted, specific marketing for very broad, non-targeted ads. Think of a shotgun instead of a rifle, or casting seeds rather than putting individual ones in the ground.
AoL goes for as many people as possible with material as broadly geared as possible.
The big question you should be asking for your business, is whether you should go above the line or below the line (more on BtL next week).
And the answer to that question is another question, or rather, two more questions:
1. Who is my audience?
If you are making something that appeals to young and old, male and female, American or Chinese, Buddhist or Mormon...you should go AtL. Examples of this include food & beverage, automotive, and medical. If your audience is 55 year old women from Buenos Aires...maybe do something more targeted.
2. Why not both?
Seriously. If you have a preferred demographic, but want to open channels to a bigger market, doing AtL and BtL can have a synergistic effect that your business may benefit from tremendously.
Bottom line: sometimes either side of the line, above and below, can have their own respective rewards.