Charms on Cyntara are trinket items that provide extra stats that your character can benefit from aside from your inventory and spells.
You can find many charms on Cyntara that can be looted from monsters or obtained through quests.
If you find an item that you think may be a useless trinket it's worth taking a look at it - if it has stats or mentions that it's a charm, then it's a charm. Further, if the item is in your inventory and you notice a small blue-ish triangle in the upper-left corner of the inventory slot - it's a charm.
You cannot however make use of an unlimited amount of charms. Charms are only effective if they're inside of the charm container and it has a limited amount of space, so choose your charms wisely depending on the situation.
A Cursed Charm is a special charm that looks like a voodoo doll. This type of charm can only be obtained from the Void Realm. Upon completion of a Void Realm, you will receive a Cursed Charm with dynamic attributes; the attributes that can be rolled are random for each completion. For more information, check out the "Void Invaders" guide.

The charm container can be accessed by clicking the button with a present box icon under your the stop button next to the inventory or by using the !charms command. You can access this container at all times. It can hold up to 4 charms at a time.

Almost all charms can be stacked meaning that you can have multiple charms (even of the same type) in the charm container to benefit from their effects multiple times. One exception to this are charms that provide health and mana regeneration - these charms will not stack; another exception are Cursed Charms.