In the world there are many devices and items of a magical nature. In general the term 'magic item' refers to all of them. Within this broad term, however, there are distinctions. Those devices or items most often thought by the general populous as 'magic items,' permanently magicked items useable by a variety of people are technically called Arcane Devices. This is to distinguish them from less permanent magical items.
The first question might be, what is a magic item that is not an Arcane Device. Firstly those magical items that are only magical temporarily are not Arcane Devices. These include the various objects used in the various Protection spells. Another example would be weapons and items modified temporarily by combat enhancement spells. While for the duration of the spell, these are magical items, they are not permanent and thus not Arcane devices. Secondly those items created on the cuff with nothing more than Engrave Magic, a spell and some form of permanency are not usually considered Arcane Devices unless of an exceptional nature.
Thirdly Object Linked items and talismans are not technically arcane devices. This includes spirit talismans, (and golem talismans) and demonic talismans. While used for magical spells and very valuable, these are still not true Arcane Devices. One common magical item often mistaken for an Arcane Device by the public, is a Wizard's Staff. While there are staves that are Arcane Devices, the traditional Wizard Staffs carried by wizards are not considered Arcane Devices, since only the person for whom the staff was created may use it.
Finally, common spell components that are reused are not Arcane Devices, allthough the public may believe otherwise. Examples of such are normal crystal balls, scrying mirrors and bowls, Evil Eye's, and similar items. There may very well be special Arcane Devices that are in the form of one of these items, but unless spells have been added to these components to give them independent powers outside the spells they are used in, they are not Arcane Devices.
Thus what is an Arcane Device? An Arcane Device is any item that is permanently magical in nature or function, independent of spells that it may be used with. An Arcane Device is created by a known set of spells and rituals and can be classified and rated (albeit somewhat sketchily in some cases).
Use of Arcane Devices:
Arcane Devices with constant effects can generally be used by anyone, unless specifically stated in the device's description. Devices which 'cast' or launch spells (like Fireballs, etc.) have key words for each spell. These key words must be said softly (or loudly if the owner is stupid) by the holder of the item, in order for the spell to function. The exception being 'intelligent' devices which can either just be told to work or may chose to do so on their own.