Inertia is a very, very subtle concept. Few people (incl myself) really have a good understanding of what is meant by inertia and why it is so perplexing. There are some great examples I've come across in the past that demonstrate, but I'll have to look them up. In the mean time take a look at the "Inertial frames" section on the wikipedia page: http://en.wikipedia.org/wiki/Inertia