.iso = images of a legit DVD, CD, in Wii hacking means games.
IOS = Input/Output System, they are little OS functioning as base of each application on the Wii,
Eg. the WiiShop Channel (current one) use IOS61,
the new HBC1.0.3 use IOS61 too,
the SysMenu3.2 uses IOS30,
the SysMenu4.0 uses IOS60,
SSBB uses IOS36,
RockBand games use IOS37,
NFS:Undercover IOS55,
Punch Out IOS53,
Animal Crossing IOS38,
Zelda TP IOS9
...and so on and on