set or get magento session

//Setting session
Mage::getSingleton('core/session')->setMySessionVariable('value');
//Getting session
Mage::getSingleton('core/session')->getMyDesignProductVariable()
//Unset session
Mage::getSingleton('core/session')->setTestingMagento();

//(Use customer or core session in frontend. Use adminhtml session in the backend.)
Core Session:- Mage::getSingleton(‘core/session’)
Customer Session:- Mage::getSingleton(‘customer/session’)
Admin Session:- Mage::getSingleton(‘adminhtml/session’)