A smartphone usually has a battery with a capacity between 1,500 and 4,000 mAh. A tablet, on the other hand, has a somewhat larger battery with a normal capacity between 4,000 and 10,000 mAh. If you choose a power bank with a capacity of approximately 10,000 mAh, you will be able to charge your smartphone between 3 - 5 times on one charge.
Most power banks today come with more than one output. 2 outputs are nice if you want to share your power with others, or need to charge your tablet and smartphone at the same time. The more amps (A) the USB port can deliver, the faster you can transfer the power to your phone from the power bank.
It is important to look at your needs when choosing a power bank. There are an incredible number of models, so you can easily get confused. A model with 10,000 mAh, 2 x 2A USB ports and a compact design will suit most people. The prices have also become so low that it is easy to find a cheap version with a high capacity from a big brand. If you want a lot of power, look at models with a 20,000 mAh battery or more.
If you have special needs, there are also models that are waterproof or have built-in solar cells for charging.