Simple. This is a big step for Fedora. In the past Fedora "Core" was written as a sort of base operating system that could be added on to. Well, Extras was added to it. But in a separate repository. And so extras existed, quietly gaining speed, size and contributors. Core was entirely managed by Red Hat. Extras was entirely community, anyone could add packages to it, including Red Hat people. Now, there is no more Extras, no more Core. It's one community operation.
So what has changed? Well the immediate changes aren't that big to the end user. Some core packages can now be linked against extras packages, that's nice. The real changes are in the community itself. By completing this merge and setting up the underlying infrastructure we have greatly increased the potential of our future. As our contribution base continues to grow so will the Fedora universe, and that's why the merger is so important.
The merger is just another number in a great list of reasons Fedora 7 will be the best release yet. Don't miss it, May 24th is when Fedora 7 Final will get released.