What's the difference between 18 A & 18 Ah on a Battery
An 18 A battery is one which is rated to produce 18 amps of current at rated voltage, which is not something all batteries can do. An 18 Ah battery can produce a current which, when multiplied by the time (in hours) the battery can supply it, equals 18. So, an 18 Ah battery can provide, let's say, 1 amp for 18 hours, or perhaps 2 amps for 9 hours, or 3 amps for 6 hours, or 0.5 amps for 36 hours. 

In comparing batteries, it's useful to talk about loads described as "C". A 1 C load is simply a current which will discharge a given battery in 1 hour. A 1/2 C load will discharge in 2 hours, and a 1/10 C current will let the battery operate for 10 hours. So, for instance, an 18 Ah battery operating at 1 C will provide 18 amps for 1 hour. Most battery chemistries, including lithium-ion, NIMH and NiCd (nickel-cadmium) will happily provide 1 C currents, but others, such as standard alkaline and primary lithium will not. 

Note that there is no guarantee that an 18 Ah battery can provide 18 amps for 1 hour, or even 18 amps for 1 second. It only implies that, whatever current the battery can provide will, when multiplied by the time available, be 18. 

Since batteries become less efficient at higher current, the Ah rating will always be specified at some nominal current, and operating at higher currents will probably provide a capacity less than the Ah rating.

For instance, let's take a standard Panasonic coin cell, BR2032 It has a nominal capacity of 190 mAh - let's call it 0.2 Ah. If you put 90 of them in parallel, you'll have 18 Ah. However, that "standard" load for these things is only .03 mA. That's right, 30 uA. So 90 of them is only rated for 2.7 mA. Trust me, such a battery pack will never, ever produce 18 A, no matter how hard they try.

At the other extreme, LiPo batteries used for quadcopters have very high C ratings. Here, for instance is a typical battery you can buy. Its capacity is only 2 Ah, but it is rated for 25 C. In other words, it will happily run at 50 amps (25 x 2), although it will only do so for about 2 1/2 minutes or less.

So, there is no guarantee that an 18 A battery will provide 18 Ah of capacity, or that an 18 Ah battery will provide 18 amps at rated voltage. This is why it is necessary to distinguish between the two.