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I think it is milliamp hours, will tell you how long they should last. I have found that most manufacturers put cheap batteries in their products and the after market stuff is better because these companies are in the business of selling batteries, not devices.
Now for cell phones and laptops (which generally use NiMH or LiOH batteries), I'd stick with OEM.
There aren't that many battery manufacturers in the world, and I guarantee the electronics companies aren't making them in-house.