Capacity and mAh
In short, mAh means "milliamp hour". It is talking about the battery life. A higher mAh means your batteries will last longer.
The mAh rating is also known as "capacity".
So a 3000mAh battery will run at 3 amps for an hour, and a 2000mAh battery will run at 2 amps for an hour until it is dead. However, this is assuming that all battery companies rate their batteries accurately (spoiler alert: they don't). Almost all battery companies inflate their ratings to sell more, but not us.
How do I know what mAh battery will work best for me? Lets break it down.
If you are a casual vaper, for instance, your best option is to find a 3000mAh battery. These batteries work best in protected devices, such as a regulated box-mod, because they provide both power and long battery life.
If you are an experienced vaper that uses a mechanical device, you will have to be more cautious. With mechanical devices you must pay more attention to the AMP rating (not mAh), because these devices use much more power than regulated ones. Typically, you will want to use a 25A or 30A continuous amperage on unregulated devices. Higher power batteries will have a shorter battery life, so usually the mAh rating for these will be around 2000mAh.
Please use a calculator like Wire Wizard if you use mechanical/unregulated mods, as using the wrong battery for these devices can result in fire, injury, or even death. If you do not understand Ohm's Law, or you're just getting into vaping, do not use mechanical or unregulated mods.
Please be informed that most battery companies lie about their Amp and mAh ratings. This can be very dangerous if you are using both regulated and unregulated devices. Always buy from trusted battery companies, like Juiced, to ensure that you know exactly what you're getting.