Definition
The letters "mAh" stand for milliampere hours. Milliampere measures a unit of electric charge, and tells a user how many electrical currents are released in an hour.
Power
The higher the number preceding the letters "mAh," the longer the battery will last. For example, if the label on the battery reads "2500mAh," it means the battery is getting 2500 milliampere an hour. Typical rechargeable batteries are rated between 1800mAh and 2700mAh, and can be purchased in different sizes.
*The 18650 2000mAh IMR battery is unprotected. (shorter usage time, but allows for higher drain) + battery is shorter (call it normal) in size.
*The 18650 3100mAh IMR battery is protected. (longer usage time, but protection can kick in when draining too much power from it) + battery is longer in size couse of the build in protection.
Most people use unprotected ones in lasers, flashlights etc.. but the choise is yours.
I like to have both.
Cheers ,sm.