This just a start to know what you’re looking for.
The ampere on an adapter (the part you plug into the socket) must be >= greater than or equal to that required by a device in order to charge. For example, most HTC phones in the past uses 5V and 1A , using a 5V/2A charger will work just fine as it adapts to the need of the phone. Using a 5V/0.5A charger will make it to charge a bit slowly, like when you charge from a computer. Using a lower A will not hurt your phone, it will just take longer.
Be aware that using a charger that’s below 5V/0.5A will most likely not charge your One M7/M8/etc.
Current phones like One M8 comes with 5V/1.5A (7.5W) charger, and soon you can buy a 15W rapid charger (US | EU) for that (it simply chargers quicker). The Quick Charger 2.0 technology used in that will need for the phone to be certified by qualcomm, the One M8 is.
The output of the charger will adapt to actual need, if it’s a 2A charger and your phone requires 1A: it will not go over 1A. This assuming it’s a good charger from a good maker. The cheaper fake chargers often gives out below what they say they do. Like 4.5V and 0.7A. Which can lead to strange behaviour while charging. 5V/0.5A just means that
So, 5V and above 2A if you’re getting a charger and might want to use it for tablets etc.
Warning: do NOT use a 12V/2A charger as that might lead to destruction.
Written more about chargers HERE