OEM mean "Original Equipment Manufacturer"
non-oem generally means a retail box but in this case it probably means they are not made by Canon - or whomever makes the Canon batteries.
Me? I wouldn't touch a non-oem battery with a bargepole. Especially in light of the recent battery recalls (laptops and mobiles). Ask yourself how likely it is Canon (or any other manufacturer for that matter) will replace your cam should a non-oem battery explode in it.
Paranoia? Probably.
