Dan Gillmor wrote a column in the Financial Times in which he tries to explain the open API phenomenon in plain English. It all may sound very simple, but it's extremely fundemental for the future of the web and everything that comes with it. On a side note, I started using Greasemonkey this week, and it would not surprise me if it will have a similar (if not related) effect on the way we use the web.
"The emerging web is one in which the machines talk as much to each other as humans talk to machines or other humans. As the net is the rough equivalent of a computer operating system, we’re learning how to program the web itself. An operating system offers programmers something called an "applications programming interface," or API. The APIs are essentially shortcuts for programmers who want to use underlying capabilities of the operating system, such as displaying text or printing, and they help products interoperate with each other."