The Apartment Protection Curse is a powerful curse cast by Rowena MacLeod on her apartment to protect it from unwanted visitors.
At some point, Rowena cast this curse on her apartment to protect it from those she didn't want visiting, including other witches. It is unknown if the curse took effect before or after her death where it served to protect her magical stash from other witches. However, Rowena specifically excluded the Winchesters from the curse's effects with it being suggested that she wanted Sam to inherit her magical stash.
After Rowena's death, Jacinda and her family came in search of the stash, but Jacinda died from the curse. Sam arrived shortly thereafter with the ghost of Eileen Leahy and was completely unharmed to the interest of Jacinda's family who sought to use Rowena's resurrection spell to bring back Jacinda, forcing Sam to get the needed materials for them from the apartment. Dean was later able to enter to burn Jacinda's body and put her ghost to rest.
The exact procedure is unknown, but the caster can choose to exclude certain people from the effects of the curse. The curse doesn't die with the caster and remains in effect. It is so powerful that even other witches can fall victim to and die from it.
The effects of the curse, which take effect within minutes, include bleeding from first the nose at which time the victim experiences blurry vision and later the eyes and mouth. The curse's effects are quick and the victim dies within seconds of feeling the first effects. The curse has no effect on ghosts.