Session management in Magento

To set the value of Magento session variable:

$theValue = 'Bhavana';
Mage::getSingleton('core/session')->setMySessionvariable($theValue);

You can retrieve the value of session variable as following:
Mage::getSingleton('core/session')->getMySessionvariable();

To unset the value of session variable:
Mage::getSingleton('core/session')->unsMySessionvariable();