Carbon Offset - what is it?

Here is an interesting ad from the newspaper which I scanned.
It is from JetStar and is marketing their 'carbon offset' programme.

This might be interesting to many of you. Click here to read more from the JetStar site...

I wonder if this is marketing hype? How would you find out?

If you have comments/ideas relating to this, please post them as comments on to this blog post.