When Joomla! 1.5.x was released, the former and original Joomla! 1.0.x came to be known as Legacy. 1.5. It can run and support 1.0.x extensions and templates. There are cases such as migrations, custom-built extensions, and others that may cause the need for this. If possible, do not turn on the Legacy mode. Try to search for either the updated 1.5 version or an equivalent version. I suspect there are many sites still running 1.0.x and at some point they will migrate to Joomla! 1.5.x. and as such, learning about legacy mode will be important for moving to 1.5.
In order for Joomla 1.5.x to support and run older components, modules, and mambots from the 1.0 family, there's a special layer called Legacy mode.
This quick recipe will show you how to activate and deactivate Legacy mode.
Log in to the Administrator Console.
Click Extensions | Plugin Manager.
Once in Plugin Manager, locate the plugin called System Legacy.
To enable Legacy mode, click the red X (assume it's off).
To disable Legacy mode, click the green check.
Legacy mode is a Plugin that emulates Joomla! 1.0. The difference between 1.0 and 1.5 extensions is great, and with this, developers and users can continue to use their 1.0 extensions. While using this you might experience some degraded performance and the site may be more vulnerable to attack, based on the extensions you are using.