At its core it is the routing of attention. You are looking at a newspaper, magazine, TV show, web site, web service (giving your attention to it) and one or more ads are trying to capture your attention and route it elsewhere. Attention is the key point of scarcity on the Internet, as it is limited by the aggregate available human time, whereas we are producing content at an exponentially growing rate. With attention scarce, paid attention routing — someone paying a price for attention — aka advertising, is here to stay.
Great description of advertising at its core from Albert Wenger in his post on paid-for vs. free services.
