What is "memory effect"?

A partially discharged battery at times does not fully discharges before recharging. It sort of "forgets" to do so. This is what is the memory effect. For e.g. if you fully charge your battery and then use only 50% of its capacity before the next recharge, eventually the battery will become unaware of its extra 50% capacity, which has remained unused. The battery will remain functional, but only at 50% of its original capacity. To deal with this, always fully discharge your battery, before recharging it. This can be done by, unplugging the device's AC adapter and letting the device run on the battery until it ceases to function. This will help your battery remain healthy.
What Are Volts and Milliamp-hours (mAH)?
These are ratings on every battery. Milliamp-hours or (mAH), is one-thousandth of an amp-hour (1AH = 1000mAH). The voltage of the new battery should always match the voltage of your original unless the batteries are different chemistries. NiMH and Li-Ion batteries have different voltage ratings, even if they're for the same device.
How long do typical batteries last?
Generally, 11/2 - 3 years, or 500-800 charge-discharge cycles. As the rechargeable battery begins to die, there is a decline in the running time of the battery, till it reaches to 50% of its running time, and you need a new battery.
Can I recycle my old battery?
NiCad, NiMH and Li-Ion batteries should be recycled. Be environmentally conscious - do NOT throw these batteries in the trash.
How long will my new battery run?
Battery run-time is difficult to determine. Actual battery running time depends upon the power demands made by the mobile phone. The use of the screen, the device and other accessories results in an additional drain upon the battery, effectively reducing its running time. The total run-time of the battery is also dependent upon the design of the mobile phone.