Charms must be in your Inventory to work. You can store them in your Stash or Cube, but they won't give you any benefits from there. So there will always be a trade off between Inventory space and Charm bonuses. If you are greedy for the bonuses and have some nice Charms, you won't be picking up too many items. Most characters will save up some nice charms in their Stash for special uses, say for taking on Act Bosses, and load up with them when the time comes, and the rest of the time make due with as few as they can, to keep more space for grabbing goodies.
Charms are magical, and like a magical item, they have at most two stats, a prefix and a suffix.
There are no Rare, Unique, or Set charms, nor superior, socketed, exceptional, low quality, etc. You see the one example of Charm stats here on the right, with the Charm in question in the shot to the left, with the cursor on it. Clvl 22 requirement, so clearly Charms will have Clvl requirements, though we don't know if these are based on their mods or the types of charms themselves, or both. Another question is what mods can be found on which charms. Unknown, but they are said to be minor, such as small bonuses to stats, resistances, and other such things.