The undead are creatures that are brought to a semblance of life after their death, in a manner that is different from resurrection in that their life is not fully restored. This is usually accomplished by binding a cursed soul to some aspect of the material world, often (but not always) their own corpse. The undead can range from zombie roltons to phantoms to much worse.
Undead are immune to physical damage from weapons that are not blessed. Clerics and paladins do not require a bless for their weapons to fight undead as long as their weapon is sanctified. Most direct damage spells harm them normally. They are immune or highly resistant to many forms of mind-affecting magic. The cleric spell Smite/Bane (302) may be more or less effective depending on the cleric's conversion status.
Corporeal undead are undead creatures that have a more solid and material existence. These can typically take normal damage from blessed weapons or sanctified weapons wielded by a cleric or paladin. Non-corporeal undead are immaterial in nature and therefore, have no physical body to hit. They are still vulnerable to attack from blessed or sanctified weapons, they are immune to instant death or debilitating wounds.