You aren't so much capturing a single skill, as much as you are taking a snapshot of a small area, and copying everything in that area, to be pasted somewhere else later. You seem to be able to copy as many ground-targeted Magic Circles at once as you can possible cram into the search-radius of Capture. I've been able to Capture and reproduce over 20 Heal Tiles, Safety Zone, and Cure from a single cast of Capture, simply because all of those abilities were placed on top of each other for Capture to grab. I have no idea if this is a bug or not, but this is how it works right now.
You can hold the Capture key to charge the skill and target a desired area, similar to multi-shot. So you don't have to be directly in front of what you want to copy.
Capture does not go on cooldown if there is nothing to copy (it also has a very pretty swirly purple-blue graphical effect).
It might seem obvious, but yes you can even capture skills that are not normally accessible at your current rank (e.g. You can Capture Barrier which is a Rank 6 Circle 3 Paladin skill as a Rank 5 Circle 1 Rogue). However, you cannot Capture skills that are not normally available to players.
There is no way of knowing what skill(s) you have captured after-the-fact, the icon for Capture does not change to match the copied skill(s), you will only know that you have captured... something, potentially a bunch of stuff.
Copied spells are forgotten when changing maps / logging off. But the cooldown is instantly reset if you lose a skill this way. The cooldown is not reset after using a copied skill and changing maps / logging off.