The 18650 battery is a lithium ion rechargeable cell which can hold about 9WHrs i.e approx 3.7V x 2.5A/Hrs. A typical AA NiMh rechargeable cell will have a capacity around 3.0 WHrs i.e approx 1.2V x 2.5A/Hrs.

Typically the 18650 cells are found in many Laptop Lithium ion batteries. A Laptop manufacturer will quote 6 cell laptop battery for example.

Both can be recharged many hundreds of times although with the AA and AAA NiMh rechargeables they can be recharged up to 500-1000 times.

So the Lithium 18650 cell would have the equivalent energy capacity of 3 AA NiMh 2500 mA/hr cells but weighs less than half the weight of the same 3 cells.

18650 cell weighs around 45 gms
3 AA NiMh cells weighs around 88 grams

The 18650 cell is also capable of delivering very high drain current values allowing up to around 3A to be drawn. This wouldn't really be possible with AAs arranged in series as the AA own internal resistance would limit the drain current. NiMh AAs are superior to Alkaline AA in this respect though.

When using 18650 it is always advisable to use protected 18650. Protected lithium cells have a little circuit board built into the cell to shut the cell down if it detects a short circuit. Without this protection the cell could overheat wilst delivering to much current and could catch fire and explode.

18650 are becoming easily available along with 18650 chargers, although possibly not from a local source. 18650 protected cells can cost around $5-13 each and the charger cost around $12-18.

http://www.dealextreme.com/details.dx/sku.5790

http://www.dealextreme.com/details.dx/sku.1251

http://www.lighthound.com/AW-18650-Protected-2200-mAh-Rechargeable-Lithium-Battery_p_20-105.html These cells are probably better quality than the Trustfire cells.

http://www.lighthound.com/Ultrafire-WF-1...r_p_6-2279.html

I normally keep around half a dozen 18650 cells available and in an emergency would recharge them with a solar charger.