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.
-Cursed
While it doesn't affect the Undead, the living will have a cursed item that acts as if it is a lower level weapon, that also cannot be taken off their equip slot. To get rid of cursed items, cast a rune of null on it, a rune of teleport, a portal rune, or a holy rune. 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 massive knockback occasionally, and is never cursed. You can get blessed items by using a rune of holy on them, obtaining blessed items from RNG and with much luck it can be found on chests