Joomla! 1.0.13 features:
- Several low-risk security fixes
- Improved password storage system
- Easier control over Register Globals Emulation
- An Itemid backwards compatibility setting
- Improved administrative session security
- Improved HTTP/HTTPS switchover support
Because Joomla! 1.0.13 is a security release, it is important that you upgrade but we strongly recommend that you take extra precautions when performing this upgrade. This release features several improvements to the password storage system designed to help protect the future security of your Joomla! powered website. These changes will cause compatibility issues with some 3rd Party Extensions, especially bridges. If your Joomla! site utilizes bridges to other applications or extensions that have their own login system such as Community Builder, Virtuemart, or others you should not upgrade your site until those extensions have also been updated.
The changes to the password storage system should be transparent to your Joomla! site's users. As users login for the first time after your site has been upgraded, their passwords will automatically be converted from the old password storage system to the new system. Because of this automatic conversion of passwords, it is important that you backup your entire database before performing this upgrade. Once the process of converting passwords has started, it cannot be reversed.
1.0.13 is available as a Full Package, which contains all Joomla! files or Patch Packages which contain only the files that have changed since previous Joomla! 1.0.x version.
I recommend that you read the whole article as it contains useful information about this release. You can find the full article here: http://www.joomla.org/content/view/3677/1/