If you use TinyMCE to edit your content inside Joomla 2.5.x (or earlier), you might have seen that TinyMCE always adds a <P> tag to sentences you wrote. How to change that?
Adding the <P> tag to the content is the the default behavior with a new installed Joomla installation. To disable that (since it might break some design elements) do the following:
1.) login to your joomla installation with a administrator account
2.) go to the "Plug-in Manager"
3.) edit the "Editor - TinyMCE" plugin
4.) In the "basic options" on the right side change the "New Lines" from "P Elements" to "BR Elements"