Using External Battery for MacBook Pro

I commute to work five nights a week, and carrying and managing my power cord is a minor pain in the butt. When charged, my Mac can be operated for about two hours, but I sometimes have three hours to play with. It occurred to me that the best solution would be some sort of external battery that I can plug in when my internal battery is close to exhausted.

However, I'm a little confused by the ads I've seen. What's the difference between an external battery and a power bank? Also, I don't want something that recharges my Mac while I sit around waiting for it to recharge. I want something I can just plug in to keep working on it.

I've also read about certain devices and/or adapters that are unsafe for one reason or another. Can anyone tell me what the name of the device I'm looking for is and how to find one that's safe to use?

Thanks.