It took me many years to discover that not all USB chargers are the same. So exactly what is it that differentiates USB chargers? Charge rates. The chargers themselves have charge rates that vary from 1A to 2.1A, and even more powerful ones that might charge multiple devices at once, such as 3.1A.
Smartphones such as iPhone 4 and 5 charge at 1A (5 watts or 1 amp) while tablets (such as iPads) and smartphones with larger screens such as the Samsung Galaxy S4, need 2.1A (10 watts or 2 amps.)
A lower charge rate charger will be able to charge your device, but it will take much longer. But you don’t need to worry about a 2.1A charger blowing up your 1A device, modern USB technology will handle the discrepancy.
Here’s a video from charger supplier Tessco that explains this in greater detail, and also defines split circuitry and shared circuitry chargers.