I've been playing and I noticed that skills have a little hour glass next to them with a number beside it. Before I thought that powers were only usable once per battle but does this mean that you have to wait a certain number of turns before the power becomes available again?
You only get one Power Card for each one that you have trained or what comes on your gear. For example, my Witchdoctor had a Charm, Token, & Ring that each gave one Valor's Armor Power Card. That meant that I would have 3 appear in my deck. She also has trained in Great Juju plus she has boots that also come with Great Juju. Thus she would have 2 of those cards show up-one from the trained spell & one from the gear.
The hourglass that is on the Power Card is a round timer. So if you have used the Overwatch Power Card that Musketeers train in, you will notice that it says 3 next to the hourglass. This means that once this card is used Overwatch will be in effect for all players (though only useful to those with Shooty weapons) for the next 3 rounds. Also some of the traps/bombs that Musketeers have, the fires left over from Privateers Big Guns Power Card, or Zeena's Rain of Fire Power Cards will have a 5 or 10 next to the hourglass. This lets you know that those bombs/traps/fires will disappear after how many rounds the card says if they are not activated. You can also mouse over the fires/traps/bombs to see how many rounds you have left to go.