basically IOS is the operating system of the Wii console...a cIOS is a Custom IOS basically a modifed version of the IOS....
thats basically all you need to know if you just wanna be a user...however if you wanna develop some stuff then by all means check this out
http://wiibrew.org/wiki/Talk:IOS/QA
that took me 5seconds to find on the Wiki am sure if you open up google and simply searched for
"Nintendo Wii IOS Tech Spec" as i just did you would find such wonderful info..