What Are Independent Olympic Athletes

While watching the opening ceremony for the 2012 London Olympics, as the athletes from different countries were parading following their flags, I was quite surprised by one group that was introduced by the commentator as Independent Olympic Athletes. Curious, I went and did some research to find out what they are, so in case anyone else was wondering, they are athltetes from countries that do not have a National Olympic Commitee, and therefore compete under the Olympic flag.