Evocable items are items which can be evoked using the v key. They offer a wide variety of spell-like abilities that have two clear advantages over regular spellcasting: they are not affected by most spellcasting penalties; most of them only require the player to increase his Evocations skill to become more powerful. All of them, except wands, must be wielded before they may be evoked.
On the other hand, their spell-like abilities are usually not as powerful as the ones a spellcaster can achieve after learning the actual spell. There is also often a hunger cost associated with using the item.
Nevertheless, they are a good choice for non-spellcasting players who wish to have a few magical options up their sleeves. Spellcasters, of course, can always use some of these to save from tapping into their own mana, or to even boost their mana (via the staff of channeling and crystal ball of energy).
Staves are a fairly common type of evocable item that can also be used as a weapon. They count as normal staves in combat and are affected by the Staves skill. They cannot be enchanted or branded.
A rod is a rare type of staff (about 10% of all magical staves are rods) that can be evoked to cast spells. They have their own mana reservoir which is recharged over time. Each spell costs an amount of mana equal to its level. Different rods have different spells to evoke.
A wand is a magical device that contains several charges of a specific spell or effect. It does not need to be wielded to be used.
Decks of cards
Decks of cards are primarily the province of worshipers of Nemelex Xobeh. Evoking a deck draws a random card from the deck to produce a certain effect.
These items can be used to summon elementals in addition to evoking other appropriately themed effects. The higher the user's evocation skill, the more likely summoned monsters will be allied with the player.
These items can be used to summon a creature or two, possibly along with other effects. In most cases, their power depends upon your Evocations skill.
These items are not magical and do not use Evocations for a successful hit. They are included for completeness, since you technically "evoke" them with the v key to attack non-adjacent monsters.