When you are customize the magento site you may be want to get current category,root category and sub category details like category ID,name,…etc.Here is a simple example.

// Load libraries
$layer 			= Mage::getSingleton('catalog/layer');
$_category 		= $layer->getCurrentCategory();
// Get Current category ID
$currentCategoryId	= $_category->getId();
//Get current category all details
$current_category 	= Mage::registry('current_category');
// Get Parent category ID of the current category  like root category
$parent_id 		= $current_category->parent_id;

If you want to load known category details ,for a example want to load category id=10, details.That is like this.

	$_category = Mage::getModel('catalog/category')->load(10);  // category ID=10
	echo $_category->getUrl();  // get category URL
	echo $_category->getUrlKey();   // get category URL Key
	echo $_category->getName();  // get category Name