Most of us rely on cell phones, tablets, and maybe a GPS unit in our daily lives and out in the field. Smart phones are basically pocket computers and it's easy to become reliant on them to augment our own knowledge. Unfortunately, they eat electricity and many of the most popular models don't have replaceable batteries. That's where an external battery pack comes in.
External battery packs range in size from units the size of a lipstick to large blocks. Modern battery technology is allowing manufacturers to cram more amp hours into smaller units, but if you want more than one phone recharge, or need to recharge more than one device, you're going to have to buy one of the larger units.
Last Fall I picked up an EasyACC 15,000 mAh pack from Amazon, and used it on my Fall deer hunting trip. It's about at the limit of what I'd call pocket-sized. The two USB ports can theoretically be used to simultaneously charge two devices, although one port puts out 1.5A and the other puts out 2.1A. However, if one of your devices requires a 2A minimum input then you'll be limited to charging one device at a time.
Aside from the big pack, I also have a smaller unit from Duracell that I picked up when I bought my first iPhone. I carry this one in my EDC bag to and from work, just in case there's a foul up during my commute and I need to give my phone a boost.