Magic items can be activated in a variety of ways. The means of activating a magic item often determines who can use it, how frequent it can be used, and how long it takes to function.
Use-activated items typically have an obvious mundane function. You do not need any special knowledge about how a sword, hat, or pair of boots works, for instance. As soon as these items are used in their typical, mundane way, the magic in them activates. Usually, use-activated items can be activated by any character class, regardless of their magical experience. Potions and oils are always use-activated. Clothes, armor, rings, and weapons usually are as well, though they may also have deeper magic that requires other means of activation.
The time it takes to activate a use-activated item depends on how long it takes to use the mundane version of the item. Magical armor doesn't function until it is fully donned. Potions usually take a bonus action to drink (or a standard action to force down someone else's throat). Magical clothing may take a standard action or longer to put on, depending on the item and the GM's discretion.
Command word items are very powerful because they can be turned on at will. Also, command word items can be used by anyone who knows the proper passphrase, regardless of class. They are also very secure, since stolen or found command word items cannot be activated without knowledge of the passphrase. On the other hand, adventurers who find command word items may also be stuck unable to use them. Sometimes, you might get lucky and the command word will be inscribed into the item in some way, though most often you will have to resort to magical means to reveal it. A common identify spell is usually not enough to do the job, though, and you may have to resort to other divination magic to come up with it. As expected, selling magic items without the command word is much more difficult, and usually you'll only be able to get a fraction of the usual price without knowledge of how to activate the item.
Unless otherwise specified, command word items usually take a standard action to activate. To do so, the user simply has to hold the item and utter the command word. It is not necessary for you to actually know the language that the word belongs to, but you have to be able to speak properly. As such, you are typically unable to activate a command word item while gagged, silenced, underwater, or similarly inhibited. Those who know the silent spell technique, can however, also use command word items silently.
Trigger activated items are similar to command word items, except that the object may have a physical trigger on it that is required to activate. This trigger may be obvious (such as the button on an immovable rod) or hidden in a puzzle of some sort. Regardless, anyone who is able to activate the physical trigger, can activate the magic item.
These items (usually scrolls) have most of a spell written on them but are only missing a few essential pieces necessary for activation. Generally, if a spellcaster belongs to a class that can cast the spell (even if at a higher level than he can normally cast) he can manage to use a spell completion item. Concentration is still required for spell completion items if the spell requires it. Somatic and verbal components are likewise necessary, and so, spell completion items can be counterspelled. They can also be used to counterspell by taking a standard action to use the item and hold the spell effect until the appropriate trigger.
If the caster can't normally cast the spell in question, he or she may be able to use spell completion items with a successful magic items check, though a failed check destroys the item and potentially backfires. Non-magic users cannot use spell completion items.
Some magic items are tethered to a part of your essence, allowing you to activate them with your consciousness. Before you can use these items, you must first successfully perform an attunement ritual. The standard attunement ritual requires one hour of concentration on the item while in your possession followed by a successful magic items check. The DC of this check can be found in the item's description, though it is usually DC 15 + twice the spell level. Some unique or powerful items may require more than the standard ritual. Details about these rituals can also be found in the item's description. If you fail any attunement attempt, you must wait 24 hours before trying again.
There is a limit to the number of items you can have attuned at one time. You start with the ability to attune one item and gain the ability to attune a second item at 5th level. Every five levels you gain another attunement slot. You cannot attune more than one copy of the same magic item. Some attunement has basic prerequisites (class, race, or ability minimums). You must meet these before attunement is possible.
Some items have multiple magical effects. When attuning to these items, you may learn some or all of the item's properties. Each effect has its own DC. When you make your magic items check as part of the attunement process, you attune to all abilities whose DC you are able to hit. Those you miss remain locked and unusable until you are able to deepen your connection with the object. As with all attunement, you can re-try attunement after 24 hours. If a future roll comes out poorly, you don't loss access to previously unlocked abilities, but you do lose all attuned spells if you ever drop attunement on the entire staff.
You can break attunement with an item at any time, but after doing so, you must re-attune to use the item again. Attunement also fades if the item is out of your possession for more than 24 hours, if it becomes attuned to another individual, or if you die.
Attuned items require a standard action to activate, and they must be in your possession to activate. However, you can activate these items without verbal commands, allowing you to do so silently, while underwater, or when tied up. Since attuned items are inherently silent and still, they also cannot be counterspelled.