Battery Question: 18650 vs CR123A What's the Difference ?

[Down1]

My flashlight has arrived today. It gives 2 battery options, 18650 or CR123A. I have never used either kind of battery before.
I am wondering about the pros and cons of each battery option.
Thanks.

[Dogman]

Quick look on the web.

http://www.candlepowerforums.com/vb/showthread.php?t=213176

[Down1]

I bought these for now.
http://www.amazon.com/Tenergy-Propel-Lithium-Battery-Protected/dp/B001W9Y4PK/ref=sr_1_1?s=gateway&ie=UTF8&qid=1285188522&sr=8-1
Will look into both kinds of rechargeables down the road.