A bit of electronics knowledge doesn't hurt.
The power supplies on the market are VOLTAGE PSUs. This means that their output VOLTAGE is regulated, not the output CURRENT. (Let's not start a discussion about the quality of these PSUs, i.e. how stable the output is.)
A 5 V PSU will provide 5 V all the eay to its maximum current rating; however, the current drawn from it depends entirely on the load. This means, for example, that a PSU that can deliver 2.1 A will only deliver 1 A to a load that draws 1 A, but it is entirely possible for some loads (such as the iPads) to "block" charging if the PSU cannot deliver the expected current (2.1 A in their case).
As for the units, mA and A are for the current while mAh and Ah are fot the electric charge (1 As = 1 Coulomb, but Ah and mAh are more convenient). So, a charger delivers A (or mA) and a battery stores Ah or mAh.