Do u want to replace an alkaline battery with rechargable one?

When u goto buy a rechargable battery, there is a lable of capacity on it (like 1000mah), while an alkaline battery has not. Really confused about what it is?
Here is the answer from http://www.all-battery.com/index.asp?PageAction=Custom&ID=20
I do not mean what they said are all true, but it can be a reference.
Q4. What does battery “mAh” capacity mean?
Answer The mAh capacity rating refers to the storage capacity available for a particular battery. A battery with a capacity rating of 1800 mAh could deliver a current of 1800mA for one hour. Higher mAh ratings for the same battery type will generally mean longer run times. This does not apply when comparing different types of batteries. This means that you may not be able to predict how long your electronic device will run just by looking at the capacity rating of a battery. When powering high drain electronic devices like digital cameras, computer peripherals, or portable music players, an alkaline battery will only deliver a small fraction of its rated capacity. For example AA alkaline batteries typically have a capacity rating of over 2,500 mAh and AA NiMH batteries have rated capacities of only 1,200 to 2,000 mAh. But when it comes to actually powering an electronic device like a digital camera, the NiMH batteries will often run the device for three or four times a long. Even comparing the capacity ratings of similar types of batteries won't often work since different manufacturers can measure battery capacity in slightly different ways. A NiMH or NiCd battery is likely to deliver much closer to its rated capacity when it's powering high drain devices. Alkaline batteries have a high rated capacity, but they can only deliver their full capacity if the power is used slowly. This means that a NiMH battery with a rated capacity of 1200 mAh can take many more photos than an alkaline battery with a rated capacity of 2,800.
PS: AAA alkaline have capacity about 1200mah