Abjurations are items that are necessary to remove curses from pieces of cursed armor or food, thereby making it usable by the player. These un-cursed items are often very nice pieces of gear that are highly sought after by players. The abjurations, of course, are often difficult to obtain. They mostly drop from high level Notorious Monsters that are usually only killable with a group of people, but they can also drop from the new area "Einherjar".Then, if the abjuration drops, only one member of the group will be able to have it. Needless to say, some abjurations can be very difficult to obtain.
As previously stated, abjurations are typically dropped from monsters that require high level alliances. Abjurations are Exclusive so they are limited to people who aided in defeating the HNMs that drop them. Some abjurations have a better drop rate than others.
The new area, Einherjar, also drops several abjurations. The exact possible abjurations (as well as whether they are specific to certain bosses, chambers, or wings) are unknown as of yet, as well as the drop rate, although at least 2 sets are obtainable only in Einherjar. Abjurations for body armor have yet to be confirmed for wings I and II.
Cursed items are crafted by high level crafters. The high quality versions of these items have a "-1" (as opposed to the typical "+1"). Cursed items cannot be equipped or used until the curse has been lifted by using the corresponding abjuration. You can find the cursed items at the Auction House, or find a crafter with the appropriate skills and commission them to craft you the cursed piece.
Go to the cathedral in Northern San d'Oria and talk with Alphollon C Meriard. He will offer to perform a ceremony to purify any cursed item you have; trade him both the abjuration item and the appropriate cursed item to receive the usable armor or consumable item.