Symbols appear before an item in the inventory, and can be any of the following.
Enchanted (+)[]
Enchantments are from Runes and also occur naturally. Any weapon or piece of armor can have several levels of enchantment from a single rune, and can have any combination of runes at different levels. The enchantment symbol is overriden when occuring on a cursed or blessed item, but the enchantment menu is still available inside the object. When blessed or cursed, the enchantments menu will also have the corresponding statement entered (blessed or cursed).
Cursed (-)[]
While it doesn't affect the Undead, the living cannot take a cursed item off their equip slot. To get rid of a cursed item, cast a rune of null on it, a rune of teleport, a portal rune, or a holy rune, or change into a face of an undead race temporarily if it's a cursed weapon. To avoid equipping cursed items, equip them to the minion (while in skeleton form) - you will be notified of the curse and can unequip it from your minion. You can also use a rune of identify with similar effect.
Blessed (*)[]
Causes weapons to occassionally smite enemies, and any blessed item is never cursed. You can get blessed items by using a rune of holy on them, obtaining blessed items from RNG, and, with luck, finding them in chests.
Blessed armor resists smiting and cancels smite damage. Since only half of normal damage is immediately applied to the victim on a strike that causes smite, this makes blessed weapons only half as effective against an enemy with blessed armor.