Evocable items are items which can be evoked using the V key. They offer a wide variety of spell-like abilities that have three clear advantages over regular spellcasting: they are not affected by most spellcasting penalties; they do not require MP to use (unless the player has acquired the MP-Powered Wands mutation and then still only for wands); and most of them only require the player to increase his Evocations skill to become more powerful. They are also not banned by Trog, unlike all spells.
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 energy and crystal ball of energy).
Staves of poison, earth, fire, air, cold and death deal melee damage which scales off of that magic skill and evocations skill. The staff of energy can be evoked using v when wielded to channel MP, regaining a few points of MP for a minor hunger cost when successful, and the success chance scales off Evocations. The unrandart staves, the staff of Olgreb and the staff of Wucad Mu, also have an evocable action with v and scale off evocations skill: the former casts Olgreb's Toxic Radiance, and the latter acts as a more powerful but more dangerous staff of energy.
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 gifted by Nemelex Xobeh to worshippers. Evoking a deck draws a card from the list of possible cards for that deck to produce a certain effect.
These items can be used to evoke elemental themed effects. The higher the user's Evocations skill, the more powerful the effect will be.
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.