While LibreOffice 3.3 is based on Open Office 3.3, there a number of key differences, including an improved Windows installer that bundles a number of extensions that typical users would not have bothered downloading from the extensions repository. It also fixes a number of bugs in Open Office 3.3, including memory and startup issues. If you want to know all the details in the new release, check out the official Feature list.
The LibreOffice project began four months ago, when the OpenOffice.org project announced it was renaming itself as The Document Foundation and was separating itself from database giant Oracle. LibreOffice may actually end up being a temporary name, as Oracle has been asked to donate the OpenOffice.org brand to the foundation. Initial supporters include the Free Software Foundation, Open Source Initiative, Canonical, Red Hat, Novell, the GNOME foundation, and Google.